I wish I shared your certainty that it was just Mourinho to blame. It would be so bloody convenient, but we've been through this twice over already and I just don't see a squad here that can challenge for the league, not under Mourinho or anyone else. We know that Fellaini is a Mourinho player, but the other big issues we're starting to see emerge – a supposed acceptance that our left back options 'will do' and a failure to improve at right back – are areas that he's on record, or those close to him are, as saying he wants to improve.
If, as is looking likely, we do go into next season without key additions that Mourinho has identified (as we did with our failure to sign Perisic last summer – regardless of the respective merits of that transfer or not) I don't see how people can argue he has 'nowhere to hide'. I think there's a systematic failure of expertise and ambition at the club and, unfortunately, Mourinho is at worst a symptom of that failure.
My very real fear, and I of course may be wrong, is that Mourinho is actually doing about as well as can be expected with the squad he's got and these grandiose hopes that sacking him will 'free up' certain players and lead to swashbuckling, trophy winning football is as misguided as when people thought it about Moyes and Van Gaal too.