He perhaps wasn't even Moyes' first choice.
But the advantage of a reunion with the Belgian midfielder is that he knows what he's getting.
So keen was Moyes to sign Fellaini when he was manager of Everton, that he borrowed a private jet to seal the deal on deadline day in 2008, according to this report from The Independent's Ian Herbert.
Moyes splashed a club record fee at Goodison Park and was rewarded with 33 goals in 177 games.
More than that, he bought a player other teams didn't want to play against.
Fellaini scored the winner when United lost at Goodison Park last season, and Sir Alex Ferguson was so worried about him in the return fixture that he asked Phil Jones to do a man-marking job.
At Everton, Moyes took advantage of Fellaini's flexibility.
Sometimes he played in the centre of midfield; sometimes he was deployed a little deeper.
He was used behind the striker on occasion, with the duty of holding the ball up to help Everton push up the field.
But with the attacking talent Moyes has at Old Trafford, it's unlikely Fellaini will be needed as a No. 10.
Wayne Rooney will be first-choice whenever he's fit, and it's also Shinji Kagawa's preferred position.
But that's not why Fellaini has been bought.
He's been signed to beef up United's midfield.
Michael Carrick and Tom Cleverley have plenty of technical ability, but they're not physically intimidating. Fellaini is.
At nearly 6'5", he's a handful.
United fans aren't going to see him marauding through midfield like Yaya Toure, but they are going to see him harry and hassle, win the ball back and keep it.
It'll be the United fans' first chance to see what Moyes has in mind for him at Old Trafford.
It's likely he'll partner Carrick in the centre of midfield. Cleverley, an ever-present so far this season, is the obvious candidate to miss out.
Fellaini is better at sitting deeper than Cleverley, who relies on sharp movement and quick feet to find a yard of space.
Alongside Carrick, Fellaini can be the destructive force and let the England midfielder be the creative side of the partnership.
Carrick will be free to concentrate on getting the ball to the front four—whether it's Robin van Persie, Wayne Rooney, Shinji Kagawa, Nani or whoever—so they can get on with the business of making and scoring goals.
Fellaini still has to win over some United fans. That first crunching tackle or first towering header should do it.