We cant know for sure, but the events since the last two years under Martinez to the present (including the trying and dismissing of three managers if Unsworth is thrown in - because I think we saw a lack of fight for him too) is disturbing.
Yes, managers are to blame too, of course they are. But senior players have gone missing way too many times in recent years and the suspicion (shared by outsiders) is that is a conscious decision.
There is a strong sense that something isn't right behind the scenes. We've seen dramatic drop-offs in performance under every manager since Moyes, which shows an alarming level of professionalism. If it happens under one manager then you could perhaps blame the manager, but to happen so consistently suggests the blame rests with the players. They've shown before a tendency to wilt when they get into positions to do something good (and the pressure thus becomes on), so I'm not convinced at all that we have a good culture in the squad. Where that's come from I have no idea.