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Sunderland boss Peter Reid told the club's official website: "It's disappointing for Kevin but it's one of those things where we thought it would be best to get the operation done straight away.
It's a fairly routine operation and the problem is on the other side of his body to where his previous injury was.
"It's difficult to put a time limit on things like this but if all goes according to plan he should only be out for three to six weeks.
We just felt that it was important to have the problem dealt with straight away."
Phillips's injury opens the way for new signing Marcus Stewart to make his Sunderland debut in Tuesday nights Premiership game at Middlesborough.