Fellaini has struggled to make an impression at United since his move from Everton and is currently out after a wrist operation.
Kompany told talkSPORT: "People shouldn't forget that it is Fellaini's first season at Manchester United.
"It's a big move and he deserves some time.
"It will sound weird because he plays for the red half of the city but I want him to get back to his best.
"He's an important player for Belgium," he added. "He's one of those special players. You just feel he can have a big impact on the team. They will need him a lot.
"It's just a matter of time really because he's got the lot."