Wayne Rooney is likely to be out for three weeks with a cut in his forehead.
The striker is set to miss England’s World Cup qualifiers against Moldova and Ukraine and could be a doubt for the Manchester derby on September 22 after being hurt in training by team-mate Phil Jones.
Manchester United manager David Moyes said: "It’s right down the middle of his forehead where it takes a wee while to heal. Probably [he will be out for] three weeks. Phil Jones lifted his boot and caught him right down the middle of his forehead."
Rooney, who missed much of pre-season with hamstring and shoulder problems, sat out United’s 1-0 defeat to Liverpool at Anfield.
And defender Jones joined the injury list, though Moyes is not sure if he will also be out of the England games.
"He had a bad twist to his ankle so I couldn’t give a definite [answer]," he added.
Moyes reiterated his determination to sign before the transfer window closes but is not yet sure if he will be able to bring in targets such as Marouane Fellaini and Ander Herrera.
He added: "We will continue working to get what we can in. Maybe in the next how many hours we will get a chance, but I can’t give you a heads up."
Moyes suffered his first defeat as United manager but insisted he was happy with the performance.
"After the first few minutes, it is probably the best we have played,” he said. “I was really impressed with how we played. I can see why we are champions."
If Moyes is correct and Rooney does withdraw from England duty, national team boss Roy Hodgson is left with Jermain Defoe, Rickie Lambert, Daniel Sturridge and Danny Welbeck as out-and-out strikers.