Bardsley sustained the injury early in pre-season and is set to miss "a number of weeks".
Boss Martin O'Neill revealed his defender had already gone under the knife, but was able to deliver better news on injured trio Stephane Sessegnon, Wes Brown and David Vaughan.
"Phil Bardsley has had an operation," O'Neill said. "That's a real blow for us."
Sessegnon returned to training following an ankle complaint at the Academy of Light on Tuesday and O'Neill hopes he will be back in action soon after the Black Cats' opening game away to Arsenal this weekend.
"Stephane has done a little bit of training this morning and hasn't felt any affects," O'Neill said.
"He hasn't been able to do that much but in the course of the next few weeks in terms of the sort of fitness level that he can attain, we hope he gets there pretty quickly.
"Wes Brown is feeling a bit better but obviously getting a bit annoyed that injuries have plagued him.
"David Vaughan's making progress and he has had an operation on his side to cure a problem. He's making good progress out there on the training field, but hasn't joined in with us yet."
Source: PA
Source: PA