Personally think he is a scapegoat. Rooney has played poorly. Fellaini has had poor games last season and the season before - if a game doesn't suit him he doesn't look great. He is also had great games and the people that said he was championship level were nearly in tears afterwards. Most of his criticism is based around 3-4 moments of a game where he doesn't look like a tidy ball players, with no high level view of his performance. He could make 8-9 headed clearances from opposition corners and give away 3 fouls on the half way line and the match day thread would fill for 15 pages about how embarrassing he is. Meanwhile one of the best managers ever goes into his interview and says he's delighted with the performance, fans here are still convinced that, based on the average 1 hour they seem him play per week, that he is doing damage to the club and his inclusion this year will mean we 'can't win anything'.
There are degrees of hating fellaini, from people that want us to play sexy football (reasonable, but mourinho should be in the firing line just as much if this is the case, if not a lot lot more) to people that I personally feel are delusional in how they view their ability to analyse football