The 26-year-old signed for United last summer from Everton for a fee of £27.5m, but he struggled for form at Old Trafford as he only made 21 appearances in all competitions.
However, he came off the bench for Belgium in their World Cup opener yesterday to score as the Red Devils overturned a one-goal deficit to defeat Algeria 2-1. Marc Wilmots used Fellaini as an attacking midfielder, whereas last term for United, he was primarily played in a more central role.
With that in mind, Jan Vertonghen told reporters after the game: "We know how to use him and I don't know whether they do in his other team.
"Marouane is always very important for the Belgian team. I know there has been a lot of pressure on him from the fans in England but in Belgium he always does well and scores goals."
Meanwhile, captain Vincent Kompany added: "Fellaini is a terrific player and any player in his first season at a new club, it can be tough. You have to grant players a couple of seasons before you can judge someone. For us, with Belgium, he has been doing it time and time again."
Fellaini has been capped 50 times for Belgium, for whom he has scored nine goals.