I understand what is being meant by 'will Fellaini be a Carrick or a Young' in terms of being able to step-up and be United standard - but there are a number of things to consider.
Being a winger, Ashley Young is expected to put up numbers, he's expected to be exciting, to show some flair, to beat a man - he's an attacker and he needs to show it. Manchester United wingers aren't about being mediocre and simply 'doing a job' - they're there to penetrate, to create and to score; to excite.
As a central-midfielder though, United have had plenty of players who have been incredibly important to us without being particularly exciting with their passing - Keane, Robson, Fletcher, even Carrick when he was partnering Scholes. Of course, Keane and Robson were 'world-class' and were very good on the ball, but I'm not sure they'd get into any teams on passing alone - and that's fine..
Fletcher most certainly wasn't, yet was considered an incredibly important player in one of the best teams United has ever had - he could get away with 'doing a job' and not being the most exciting, because we had other players capable.
Fellaini can do everything that Fletcher can, and some things better. He has got a bit of flair to him, can carry the ball power, and will score goals - the only part of his game which is lacking is his creative passing from deep - but since he can link-up play well, run with the ball, and hold up play - there's no issue for me here. As long as we have players like Carrick, Rooney, Kagawa, Nani in the team (fit and in form, which has lacked lately admittedly) then it shouldn't matter that he can't pull out wonder balls.
He'll never be considered world-class (or even 'top-class'), but that doesn't mean he can't become one of our most important players. He'll definitely do a job for us defensively, but I'm confident he can use some of his other qualities in our attack - qualities that we've been lacking.
A world-class team doesn't need 11 world class players, we've seen this time and time again.