Even though Manchester United managed to qualify for the semifinals and will only play against Coventry, of course, it is certain that Manchester United will also qualify for the final match. But unfortunately, in the final match Manchester United will face Chelsea or Manchester City but yes, if for example in the final match Manchester United meet or play against Manchester City, then I personally am not sure Manchester United can beat Manchester City. Therefore, even though Manchester United is certain to play in the FA Cup final, but there is still no strong guarantee for Manchester United to get the FA Cup trophy.
But indeed for the semi-finals at least they are possible to win because their opponent is Coventry City who surprisingly can be in the semi-finals because their opponents are still quite comparable to the quality of the players, it's just that for the semi-finals against Manchester United it allows them to fall and have to end their hopes of being in the final.
But I agree with what you said because Manchester United's task is still quite heavy especially to see the final. They are favourites in the semi-finals but not necessarily for the final because looking at the other pots in the semi-finals there are Chelsea and Manchester City which are indeed 2 big clubs. The ideal final for the FA Cup might be Manchester City facing Manchester United but certainly it will be a tough test for Manchester United in giving the trophy that they can still fight for but it is definitely difficult to get.