Fellaini saw a specialist on Monday who confirmed he would need an operation on the ligament injury that kept him out of Saturday's game against Sunderland.
But the timing will be intriguing.
Unlike a knee problem or ankle knock, Fellaini can play on with a wrist injury with the help of a protective cast.
He's likely to play for Belgium in their World Cup qualifiers in Croatia on Friday and against Wales in Brussels on Tuesday.
It's understandable that the 25-year-old wants to play, with his country on the verge of qualifying for its first major tournament since 2002.
But the decision about when Fellaini goes under the knife ultimately rests with United.
Moyes has two options.
He could send Fellaini to the surgeon immediately after the international break.
The plus side, however, is that the operation will be out of the way as quickly as possibly and he would likely return for the busy festive fixture list.
But Moyes has reason to want to delay the procedure, too.
On Tuesday, Sky Sports reported that Fellaini's operation wouldn't be scheduled until later this year.
That would leave the £27.5m midfielder free to help guide United through the remaining Champions League group games.
It would also solve a problem in midfield until Moyes can dip into the transfer market.
It might appeal to Moyes to let Fellaini nurse his wrist through October, November and December, then let him rest and recuperate in January and February with Ander Herrera, for example, signed to fill the void.
Last week, Moyes warned fans not to expect new arrivals in January despite a disappointing recruitment drive this summer.
But Fellaini's injury has changed things.
It would leave him with only three recognised central midfielders—Michael Carrick, Tom Cleverley and Anderson—a situation that convinced him to push the boat out to sign Fellaini from Everton on deadline day.
Some United fans weren't too disappointed after news of Fellaini's injury broke.
But Moyes is already short of options in midfield and can't do without him for very long.
Delaying Fellaini's operation until the end of the year would suggest Moyes wants to have the Belgian available until he can bring in cover.
And it puts Herrera, the one who got away this summer, back at the top of his wish list.