Tim Howard will be out for several weeks after David Moyes revealed that the American has fractured two bones in his back.
Howard sustained the injury against Oldham Athletic in the FA Cup fifth round replay 10 days ago.
Jan Mucha made his Premier League debut in last weekend’s 3-1 win over Reading and the Slovakian international continued in goal against Wigan Athletic.
Moyes said: “Tim is out against Wigan. He has broken two bones in his back. We don't think he will be out for the season. I read that (Newcastle's Fabricio) Coloccini could be out for seven weeks (with a similar injury) but we are not sure how long Tim will be out for because, obviously with a goalkeeper, he has to dive around and hit the ground.
"I have let him go away for a few days. It might be that, if everything goes well, he misses next week's game as well [against Manchester City] and then because of the international break he might be close after the rest because that would be four weeks.
"He has two fractures, but they are not bones that are supporting anything. It is off the spine. They are not weight-bearing. It just needs a bit of healing time. It is not like a pure fracture where you need to wait to look at it. It is a question of whether he can cope with the pain. The bones will heal themselves but it's not good."
Howard is joined on the sidelines by Phil Jagielka, who was forced off last weekend against Reading with a gashed foot.
Moyes remains hopeful that he will return soon. "We think it won’t be too long before he is back," the manager said at his Friday press conference. "Hopefully we get him back quite quickly.” Tony Hibbert remains a long term injury absentee as he recovers from calf surgery.