Manchester United manager Louis van Gaal refused to put a time-frame on how long Wayne Rooney will be absent for after confirming his captain had suffered a knee injury, with reports suggesting it could be up to two months.
A number of national newspapers reported the 30-year-old striker had sustained knee ligament damage that could sideline him for a couple of months, handing United a heavy blow in their pursuit of a top-four finish in the Barclays Premier League.
Rooney was not included in the travelling squad United announced on Wednesday morning for Thursday's Europa League last-32 first-leg clash at FC Midtjylland.
And Van Gaal said in his pre-match press conference: "We have 13 players injured and Wayne is one of them.
"It's difficult to say but he's injured in the game versus Sunderland and has a knee problem. We have to wait and see how long that takes. I cannot say.
"He scores a lot of goals so he is very important for us. We know that but we have to cope with that.
"Certainly in 2016, he has scored and that's why we had a good run in matches until Sunderland. But we have to cope with that. Everybody can be injured and also the captain can be injured."
If Rooney is facing a sustained period on the sidelines, it will be a significant setback both to his club and country.
United are currently fifth in the Premier League, six points adrift of fourth-placed Manchester City with 12 rounds of top-flight fixtures to go.
And Van Gaal, whose position has been under increasing scrutiny, admitted after Saturday's 2-1 loss at Sunderland that winning the Europa League was looking the most viable route into the Champions League for his side.
Meanwhile, England have a double-header of international friendlies next month against Germany and Holland as they continue their preparations for this summer's Euro 2016 finals.
Three Lions head coach Roy Hodgson said on Saturday that skipper Rooney - England's all-time record goalscorer - will be ''a player we need to depend upon'' at the tournament in France.
Rooney has netted three times in four appearances for England this season, and is United's top scorer this term with 14 goals in all competitions.