The Black Cats full-back will miss the start of the new Premier League season after requiring surgery on an injury sustained earlier in pre-season.
The former Manchester United full-back, joined the Black Cats in 2008 and has since gone on to make 130 appearances for the club, cementing himself as a first-team regular.
Martin O'Neill expressed his disappointment that the 27-year-old required an operation on an injury sustained in an earlier pre-season fixture and thus, will miss the start of the new season.
"Phil Bardsley has had an operation," O'Neill told reporters. "That's a real blow for us."
Elsewhere, there was more positive injury news for the Stadium of light boss as he confirmed improvements in the respective fitness' of Stephane Sessegnon, Wes Brown and David Vaughan. However, all three will be unavailable for their opening game against Arsenal.
|BLACK CATS LOSE A LIFE|
|6.50||Arsenal are 6.50 with bet365 to beat Sunderland 1-0|
"Stephane has done a little bit of training this morning and hasn't felt any affects," O'Neill added.
"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."