It is the reasons why he does not rate Nisbet that can be passionately questioned - without necessarily making his reasons wrong.
From what I have heard Nisbet had a lot of coaches 'not rate him' simply because of his physique. When someone gave him his chance he proved the rest wrong.
Popa obviously does not rate Nisbet and I think that is a blinkered and closed mind sort of rating - but it may simply come from Nisbet not being the type of player he wants. He is, after all, building a defensive, highly regimented system rather than a free flowing creative team. The strengths Nisbet brings to to the attack do not really suit playing in behind an isolated single forward like Popa tends to setup. Nisbet makes the various parts of a team work better in attack - that is not what Popa wants based on everything we have seen.
To use any attacking player like Nisbet effectively, Popa would either have to rejig his whole forward playing game into a cohesive attacking unit, or else jam Nisbet in a non-functioning role/position like he did when he had Irankunda out of position and have him come up useless.
I can understand why Popa does not play Nisbet and I deplore his reasoning.
As a side note - I acknowledge that Nisbet is also very good defensively even though I have concentrated on his attacking role here.