  Recent experiments performed in neutron-rich copper isotopes have revealed a
crossing in the nucleus $^{75}$Cu between the $3/2^-$ and $5/2^-$ levels, which
correspond to the ground-state and the first excited state in isotopes with
mass number below $A =75$. Due to the strong single-particle character of these
states, this scenario can be investigated through the analysis of the proton
spectrum provided by mean-field models in nickel isotopes with neutron numbers
between $N$=40 and $N$=50. In this work we show that the aforementioned
crossing is mainly driven by the mean-field provided by the effective
nucleon-nucleon and spin-orbit interactions. We also analyze the impact of the
tensor interaction, and find that in some mean-field models it is essential to
reproduce the crossing of the 2$p_{3/2}$ and 1$f_{5/2}$ proton single-particle
levels, as in the case of the SAMi-T Skyrme force and the D1M Gogny
interaction, whereas in other cases, as for example the SLy5 Skyrme force, a
reasonable tensor force appears to be unable to modify the mean-field enough to
reproduce this level crossing. Finally, in the calculations performed with the
so-called simple effective interaction (SEI), it is shown that the experimental
data in nickel and copper isotopes considered in this work can be explained
satisfactorily without any explicit consideration of the tensor interaction.
