Fellaini has struggled to adjust to life at Old Trafford since joining from Everton last summer, but Wilmots believes that the midfielder has been unable to make an impact while playing in a deeper role under David Moyes, who was sacked last week.
"I don't understand why Manchester United bought him to play him in a system of two number sixes," Wilmots told members of the Belgian media. "Marouane is a box-to-box player.
"I have no doubts about Marouane. The World Cup is a good chance for him to take revenge."
Fellaini was left out of the United squad for last Saturday's 4-0 victory over Norwich City.