Smalling suffered the injury while training at United's Carrington training ground after recovering from a thigh injury that he suffered at Euro 2012.

"He got an injury on the Wednesday and, straight away, we decided to have the operation so he's out," United boss Sir Alex Ferguson said.

"It's difficult to say with a metatarsal but it could be 10 weeks.

"He's not carrying any luck, the kid, but he's young and we've done the right thing in getting it done immediately and getting him back on the road as quickly as we can."

Smalling is not expected to feature again for United until the end of September.