The former Manchester United youngster picked up the injury in the first half against fellow relegation strugglers Hull, but played on until the 88th minute.
Sunderland manager David Moyes says that it's a "real shame" that the Northern Irishman will be out for the rest of the campaign.
"Paddy was starting to understand more about the club, the supporters and also the general game, so it’s a real shame for the boy.
"He’s really keen to do well and he works really hard at his game to try and improve, so we’re incredibly disappointed because we want to try and build a young team going forward and Paddy was part of that".
He added: "I think he'd just started to find his way in the Premier League - even though he had experience at Manchester United - and the games he was having were bringing him on and giving him confidence.
"He probably won’t have his operation for over a week because he’s got a cut on his knee and they can’t operate while there is an open wound, so we have to wait until that heals so it might be a week or two.
"We won’t see him again this season and all we can hope is getting him right for the start of next season".