London - Manchester United captain Wayne Rooney is set to be sidelined for two months with a knee ligament injury, according to British media reports on Wednesday.
Earlier, the club confirmed Rooney, 30, would miss their Europa League game at FC Midtjylland on Thursday.
England striker Rooney was not in manager Louis van Gaal's 18-man squad which travelled to Denmark on Wednesday for the first leg of their last-32 tie.
"Wayne Rooney is a notable absentee from the flight and the manager is expected to confirm why his captain has not made the trip during his pre-match press conference at 1700 GMT," said a statement on United's website.
Also read: Rooney out of Europa League match
Afterwards several British national newspapers said Rooney would be out of action for two months with a knee injury.
If Rooney's absence is as lengthy as has been suggested, it will be a setback for both United and England.
United's 2-1 loss to Sunderland last weekend left them fifth in England's top flight and six points off the top-four spot, the club's "minimum requirement" in the under-pressure van Gaal's words, that would secure entry into next season's Champions League.
They have 12 rounds of Premier League fixtures left in which to overhaul fourth-placed local rivals Manchester City, who have already announced that Bayern Munich boss Pep Guardiola will be their new manager next season.
Defeat in Denmark would merely add to the pressure on van Gaal, who has said winning the Europa League now offers United their best chance of qualifying for the Champions League.
Meanwhile there is ongoing speculation that former Chelsea manager Jose Mourinho is set to replace Dutch boss van Gaal at Old Trafford come the end of the current campaign.
England also have two friendly internationals against Germany and the Netherlands next month ahead of their participation at the Euro 2016 finals in France.
Roy Hodgson, the England manager, said on Saturday that his skipper Rooney, England's leading all-time goalscorer, will be a "player we need to depend upon" at the Euros.
Rooney is United's top scorer this season with 14 goals in all competitions and has found the back of the net three times in four appearances for England this term.