Jose Mourinho has confirmed that he was forced to take off Marouane Fellaini during Manchester United's 2-0 defeat at Tottenham on Wednesday, 31 January, after the midfielder suffered a fresh injury setback.
The Belgium international missed the busy Christmas period due to a knee injury and only returned to action in the FA Cup win over Derby County on 5 January.
Fellaini had since made two further appearances, but the United midfielder could be set for a new spell on the sidelines after being forced off in the defeat against Tottenham.
The midfielder came on for Jesse Lingard in the 63rd minute of the game but had to be replaced by Ander Herrera only seven minutes later.
"He came off because he had something in his knee," Mourinho confirmed in the press conference following the defeat at Tottenham.
"Unlucky for him and unlucky for me. He came on with enthusiasm, but he has an injury in his ligament. It's always the same thing. It is the lateral ligament. I don't think it's a big, big thing but he was immediately feeling something and telling us he couldn't carry on."
Paul Pogba was also replaced by Juan Mata in the 63rd minute of the game, with United already trailing by two goals by then.
However, Mourinho allayed injury fears over the France international after pointing out that his substitution was tactical.
"No [Pogba wasn't injured], we just changed for Fellaini to try different qualities with different players," the United boss added.
Mourinho also lamented the "ridiculous" defensive errors made by his side as Christian Eriksen scored the opener after just 11 seconds before Phil Jones doubled Tottenham's lead with an own goal.
"It's a performance that starts with a ridiculous goal. That goal after a few seconds has a big influence in the game because Tottenham can play the way they like to play," the United boss admitted.
"In the end, we played against a very good team. Both goals were ridiculous goals, also the second was unlucky. The first goal is an accumulation of mistakes in spite of the fact that everything happened so fast. My players have watched the Tottenham kick-off many, many times.
"They know the way they do it and they also know [Harry] Kane normally starts in a position in front of the line to win some metres. We didn't control it, lost the second ball on the ground and it was a really, really bad goal.
"We reacted and had a good reaction for 10 or 15 minutes with a good approach to the box and crosses in the face of the goal. But I think the second goal, mentally, killed us and it gives Tottenham the possibility to play the way they like to play.
"We tried to not let them play that way but, at 2-0, it was in their hands. A team to win matches needs to score goals and also defend well – on this occasion, we were very bad. Lukaku could score and make it 2-1 to bring the team into the game, but also, Tottenham could have scored more goals on the counter-attack."