Paul Scholes has suffered medial ligament damage and will be out for approximately 10 weeks.
The news came at lunchtime on Wednesday, the day after Scholes had been stretchered off in United's 3-0 win over Aalborg BK in Denmark.
Scholes wasn't the only casualty. Wayne Rooney and right-back Rafael were also substituted for medical reasons during the match. But Scholes was the first injured and, it would seem, the most serious.
"There were a lot of plus points but we lost Paul Scholes," Sir Alex said after the final whistle.
"We had one or two other injuries," the boss added. "Young Rafael got injured and Rooney has an ankle problem. He felt a weakness in it when he went to receive a ball and we took him off straightaway.
"So there was maybe a cost to pay for the victory. Nonetheless we have four points now and it's where we want to be."