“It is a bad blow for the boy because he missed the end of the season with a groin injury,” Ferguson said after United’s pre-season friendly draw with Ajax Cape Town on Saturday.
“We thought we had him on the right road to recovery and he gets another injury, which is bad news. He got it in training on Wednesday, was diagnosed on Thursday and we decided to operate last night.
“Normally with a metatarsal you are looking at between 10 weeks and three months. He is only 21. He is young. Hopefully he gets over it.”
Jonny Evans is also due to miss the start of the season, but Nemanja Vidic is expected to return to fitness, while Rio Ferdinand and Phil Jones provide further options. Still United cannot afford anymore injuries to its central defenders before the season starts.