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The San Francisco 49ers have seen a significant amount of roster turnover since the end of the 2014 season. With players like Patrick Willis, Justin Smith and Chris Borland retiring, and longtime 49ers like Frank Gore, Michael Crabtree and Andy Lee plying their trade for other teams in 2015, the 49ers will look distinctly different from how they have the past three seasons.

With so much turnover, it’s good to take an overview of the roster. Where are the 49ers strongest now, and where is their weakest point? What can they hang their proverbial hat on, and what will they have to scheme around?

Let’s pick one group each for the best and worst positional unit on the 2015 49ers.

Best Unit: Linebackers

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How many teams in the NFL could lose a potential Hall of Famer like Willis, a Rookie of the Year candidate in Borland and a useful depth player like Dan Skuta and still boast, at worst, a top-10 linebacking corps?

It’s a tribute not only to the depth the 49ers have at the position, but also their recent and continual success at finding players to fill holes and develop depth that, even with so much talent leaving the franchise, the linebackers stand supreme.

The unit revolves around two major question marks, but I expect both of them to end up in the positive column for the 49ers by season’s end.

The first major question mark is NaVorro Bowman’s knee. Bowman missed the entire 2014 season after suffering a torn ACL and MCL in the NFC Championship Game the year prior. It was a horrific injury, and there’s some questioning as to his ability to fully recover.

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Some have compared Bowman’s injury to the injury suffered by Marcus Lattimore that ended his career before it could begin, but it’s important to note that Lattimore’s injury was much more severe. Lattimore tore his ACL, PCL and LCL, in addition to dislocating his patella.

Bowman, by comparison, got off easy—there was no major dislocation of Bowman’s kneecap, and he still had two of his four ligaments holding strong while he was recuperating. It was significant enough to cost him the entire 2014 season, but his career isn’t in jeopardy.

He’s still wearing a brace on the knee, which annoys him to the point where he suggested he would “light it on fire” when the training staff tells him he can discard it. It is limiting his quickness and mobility at the moment, but that’s the required trade-off to add extra support as he works the knee back into game shape. The fact that he’s pain-free at this point is a tremendous sign, even if he hasn’t yet regained full explosiveness yet.

Honestly, the major sticking point in his recovery is the mental aspect of the game—getting back into game speed after missing an entire season. The Bowman at the beginning of the 2015 season won’t be the same Bowman from the end of the 2013 season.

However, with every early report from this offseason looking strongly favorable, it’s seeming more and more reasonable to assume Bowman will be back at full strength by the end of 2015.

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That’s basically adding an All-Pro linebacker to the roster, which would be one of the greatest moves for any NFL team this offseason. Even with the uncertainty surrounding his knee, Pro Football Focus gave him its coveted “elite” status; the only other inside linebacker in the entire league to earn that honor was Luke Kuechly in Carolina. Even 80 or 90 percent of 2013 Bowman is still one of the best players in the league.

The other major question mark is the return of Aldon Smith. After serving a nine-game suspension last season, Smith wasn’t quite the same force he was in his first three seasons, only recording two sacks as he worked himself back into action.

Still, that brings his career sack total to 44 in just four seasons. Only 11 players have had more than that in their first four seasons, and four of them are Hall of Famers—Reggie White, Derrick Thomas, Richard Dent and Bruce Smith.

Aldon Smith’s 0.88 sacks per game are fourth all-time through a player’s first four seasons, behind only White, Thomas, Shawne Merriman and J.J. Watt. Smith, in other words, is one of the greatest pass-rushers the NFL has seen since the league started recording sacks in 1982.

Smith’s problem has never been his performance on the field; it’s been his actions off it. That nine-game suspension was due to a plethora of incidents, including DUIs and weapons charges. Add in his five-game rehab stint in 2013, and Smith’s off-field issues have cost him nearly a full season.

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At this point, Smith has been incident-free for an entire year. He’ll be able to fully participate from day one of training camp up through the regular season. That should alleviate some of the rust issues he showed at times even when he was active last season.

If he can stay out of trouble off the field, the 49ers will be adding a top-five pass-rusher to their defense. A full season from Bowman and Smith has the potential to radically improve the defense's fortunes.

They’re just the headliners, too. While calling the linebacker position the strongest depends on Bowman and Smith rebounding, this isn’t a two-man group.

Aaron Lynch was the 49ers’ most promising rookie for the first half of last season, before Borland stepped into the lineup and blew everyone else out of the water. Lynch recorded six sacks as a rookie last season, third-most among all rookies, trailing only Aaron Donald and Jonathan Newsome.

That’s an impressive number, considering he was battling for snaps with Smith and Ahmad Brooks by the end of the year. He was a one-dimensional player last year, simply excelling at rushing the passer, but he wasn’t a liability anywhere else. If he can round out his game, he’s going to be a steal coming from the fifth round of the draft.

Michael Wilhoite had to step into the starting lineup last year for Bowman and will do the same for Willis this year, and he was up and down. Ceding the dime-package looks back to Bowman will help, and Wilhoite hopefully won’t top 1,000 snaps for the second straight season.

Wilhoite may end up the worst defensive starter for the 49ers in 2015, but he’s solid in coverage and fairly reliable—not bad for a “worst on the defense” moniker.

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Brooks is also still around. The 2013 Pro Bowler had issues with coaching last season, being benched and benching himself at various points during the season. Couple that with his high salary and his lingering connections with Ray McDonald’s various assault charges, and he’s not fully guaranteed to make the roster.

Brooks is still a decent player, though—he tied Lynch for the team lead with six sacks and was strong in run defense. He should lose snaps to Lynch if he stays on, but he’s still a good player.

The diamond in the rough, however, could be Eli Harold. The third-round pick out of Virginia is my favorite pick the 49ers made in this year’s draft. He's an explosive player with developed pass-rushing moves for a rookie, and in the worst-case scenario, he's a situational pass-rusher.

Harold will have time to develop his run defense as he learns behind Smith and Brooks or Lynch, but don’t be shocked if he’s used like Smith was in his rookie season, coming off the bench to terrorize quarterbacks on third down. Stick a pin in Harold; he might be a future star.

You can point to the three-headed rushing attack of Carlos Hyde, Reggie Bush and Kendall Hunter for best positional group if you’re concerned about Bowman’s knee and Smith’s off-field antics, but Bowman and Smith are, when fully available, two of the top players in all of football. Having them both back in the positional group for a full season makes the linebacking corps the 49ers’ strongest point.

Worst Unit: Quarterbacks

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Sadly, the 49ers’ biggest weakness might be at the most important position in football.

It’s no secret that Colin Kaepernick had his worst season as a pro last year. He set career lows as a starter in yards per attempt, quarterback rating and QBR, while seeing his interception and sack total rise. He ended up as the 28th-rated quarterback on both Pro Football Focus and Football Outsiders—a far cry from the dynamic player who led the 49ers to a Super Bowl appearance two seasons ago.

When ranking all 32 starting quarterbacks, Andy Benoit of Sports Illustrated put Kaepernick 21st, saying his “raw tools are startling, but so are a lot of his decisions.” His footwork is questionable, he sometimes seems to lock on to a receiver and misread coverages, and he fails to put touch on short throws.

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However, he was not helped by his teammates or play-calling last season. The 49ers went away from a number of the read-option and rollout plays that Kaepernick does so well. He’ll likely never be a top pocket passer, but his mobility and ability to make plays on the run make him a threat. Greg Roman shoehorned him into the pocket more in 2014 than in previous years, to his detriment.

Kaepernick's offensive line was often injured and was responsible for a significant number of his 52 sacks, with the departed Mike Iupati and Jonathan Martin particularly to blame. His longtime safety blanket in Vernon Davis suffered a massive regression and a case of the drops. Even the normally sure-handed Anquan Boldin had 10 drops last season, per PFF, as did Michael Crabtree.

The great quarterbacks can keep all those numbers down and raise the level of performance of players around him, but additional help would have been nice.

Kaepernick definitely played poorly last season, but there are reasons to believe he’ll improve in 2015. If we knew for a fact that Kaepernick would play all 16 games for the 49ers next season, I would not have selected quarterback as the 49ers’ worst unit. We could talk about Davis’ regression pushing the tight ends down, or the difficulty in replacing both Justin Smith and McDonald on the defensive line.

However, we can’t guarantee a player will play an entire season. If Kaepernick is forced out of the lineup, be it by poor play or by injury, the 49ers do not have a real option behind him.

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Blaine Gabbert is arguably the worst backup quarterback in the NFL. That’s a fairly bold claim, even in the division, with the Cardinals planning on using Drew Stanton to back up Carson Palmer, but I stand by it.

Most of the worst backups in the league are simply unknown quantities; we’ve seen what Gabbert can do when given the keys to a team. Gabbert was arguably the least successful quarterback of the past decade.

In his three years as a starting quarterback for the Jacksonville Jaguars, Gabbert put up a 5-22 record. That is the worst record for any quarterback with at least 27 starts over the past four seasons by a wide margin, with Josh Freeman’s 11-24 record coming a distant second.

Gabbert’s quarterback rating of 66.8 is the worst in the league over that time period. His 5.61 yards per attempt is the lowest in the league in that time, as is his completion percentage of 53.19.

He threw a touchdown once every 18.13 passes, which is actually only second-worst since he entered the league, ahead of Geno Smith. His 17.38 passes between interceptions is his strongest selling point, being a better total than Smith, Freeman and Mark Sanchez.

Gabbert struggles with accuracy when asked to go downfield, making him a particularly poor fit for new free-agent acquisition Torrey Smith. He also handles pressure incredibly poorly, making him an undesirable fit for a line that gave up 52 sacks last season.

Gabbert doesn’t have the arm strength needed to fit the ball into tight spaces, he doesn’t step into his passes and his footwork isn’t particularly good, either.

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Yes, playing on the Jaguars necessarily depressed his numbers. He didn’t exactly light the world on fire during last season’s preseason, however, completing just 46.8 percent of his passes and putting up a 54.0 quarterback rating against the second- and third-team defenses of the league. Only Case Keenum threw as many passes as Gabbert did and had a lower quarterback rating.

I wouldn’t peg quarterback as the 49ers’ worst position if: Kaepernick had played at a league-average level last season; Kaepernick was guaranteed to play every snap this season; the 49ers had used a draft pick on a rookie or brought in a free agent like Brian Hoyer, Shaun Hill or Josh McCown.

But couple Kaepernick’s poor 2014 season with the specter of having to rely on Gabbert should anything happen, and you have a shaky situation. It’s not the worst quarterback situation in the league by any means, but it’s the worst position the 49ers have as they enter the 2015 season.

Bryan Knowles is a Featured Columnist for Bleacher Report, covering the San Francisco 49ers. Follow him @BryKno on Twitter.