  The LHCb has discovered two new states with preferred $J^P$ quantum numbers
$3/2^-$ and $5/2^+$ from $\Lambda_b$ decays. These new states can be
interpreted as hidden charm pentaquarks. It has been argued that the main
features of these pentaquarks can be described by diquark model. The diquark
model predicts that the $3/2^-$ and $5/2^+$ are in two separate octet
multiplets of flavor $SU(3)$ and there is also an additional decuplet
pentaquark multiplet. Finding the states in these multiplets can provide
crucial evidence for this model. The weak decays of b-baryon to a light meson
and a pentaquark can have Cabibbo allowed and suppressed decay channels. We
find that in the $SU(3)$ limit, for $U$-spin related decay modes the ratio of
the decay rates of Cabibbo suppressed to Cabibbo allowed decay channels is
given by $|V_{cd}|^2/|V_{cs}|^2$. There are also other testable relations for
b-baryon weak decays into a pentaquark and a light pseudoscalar. These
relations can be used as tests for the diquark model for pentaquark.
