That's what Red Devils boss Roberto Martinez has confirmed amid concerns the midfielder is set for a spell on the sidelines.

Fellaini linked up with Belgium 10 days ago ahead of their World Cup qualifying fixtures against Bosnia and Herzegovina and Cyprus.

He was forced off in the first of those games when he landed awkwardly after challenging former Manchester City striker Edin Dzeko for a ball in the air.

Belgium initially stated Fellaini would be out of action for two weeks - ruling him out of United's showdowns with Liverpool, Benfica and Huddersfield at the very least.

Martinez has admitted he's not sure how long a stint the 29-year-old may be facing in the treatment room though.

Speaking ahead of Tuesday night's showdown with Cyprus, the Belgium boss said: "Marouane Fellaini, it has been confirmed that he has got ligament damage.

"He has gone back to Manchester United. It was a very unfortunate injury for Marouane. I don't think the state of the pitch played any part.

"A collision between two strong players, very committed that they want to win the ball, and unfortunately Marouane ended with a damaged ligament."

Martinez added: "Marouane is a very quick healer but obviously now we had the scan and it is damage in the ligament.

"Damage could be very little or very big.

"I think the next few days, we will have a clear idea of the final state of the injury. The first 48 hours, you have still got the swelling.

"We don't know at this stage [how long Fellaini will be unavailable for].

"I think with Marouane, sometimes you expect him to be out, but he is a really quick healer."

United are already without midfielders Paul Pogba and Michael Carrick for this weekend's trip to arch-rivals Liverpool (Saturday, 12.30pm).

Jose Mourinho's men are currently second in the Premier League having collected 19 points from their opening seven games.