I suppose saying he's never been great is necessarily a criticism and the phrase is figurative. i.e. the expectation isn't that he's capable of greatness but rather that he's a good player for us, but as yet his showings have never reached that in all his minutes, although the usual asterisks are attached that stop me short of saying he couldn't turn it around or is just a poor player.
The training theory is just that, a theory, but it's kinda logical right. How can you have this guy playing superbly in training but then nobody wants to play him. That's kinda farfetched compared to him not impressing for whatever reasons. Whether they're physical, footballing, personality, adjustment to the club, god knows.
But whatever it is he hasn't convinced two managers he should play. What can you take from the sequence of events expect that the managers don't see him as meriting a place, if we want to rephrase it away from training, it is just factual to state that by whatever mechanisms there are prior to matchday he falls foul of them. Now people try to blow a hole in this by saying "well how can we trust the managers", but actually managers have a degree of self-preservation that makes this difficult to reconcile. Hard to imagine not lumping in a player on the sidelines you have any degree of hopes for.