Well that's why I used him as an example. Carrick and Young were bought at similar ages to Fellaini off similar clubs for pretty high fees, one went on to up his game and be an integral part of the team, the other was shown up for what he really is. Fellaini for me could go either way.
Young has always been a mind boggling signing for me though, especially for the price we paid. Not at one point before or after the signing did I think he was good enough for us.
Carrick was already a very good and consistent player before he joined us. Like Fellaini, he wasn't seen as anything special and the fee was considered too much. I have a similar feeling regarding the Fellaini signing, I don't think it's a simple case of it going either way at all.
I mean, I'm sure there's perfectly valid reasons as to why you might think that he'll not be a success here, but he has a much better chance of being so than Ashley Young IMO. That much is clear to me.