The left-back was taken off after 50 minutes of the 3-3 draw against city rivals Liverpool on Saturday after he failed to recover from a coming together with Jordan Henderson in the first half.
A scan has now revealed that the England player has suffered a fractured toe, which could rule him out for between 10 days to six weeks.
“We need to treat that now and see how it develops. It could mean anything from missing six weeks to being back in 10 days because it depends how the fracture heals.
“It is in such a specific spot and we are going to see how it [the injury] reacts.”