The 26-year-old suffered ligament damage while on international duty with Scotland against Wales, and will now target a return to full fitness for the start of next season
|DERBY WIN IS A SMALL STEP - DI CANIO
|6/1||Sunderland are 6/1 with William Hill to be relegated
The club have not yet set a time-frame on how long Fletcher's recovery period will be, but confirmed that he had undergone a procedure on his injured ankle on Monday.
Since the striker's injury, the Mackems have replaced manager Martin O'Neill with Paolo Di Canio and gone on to beat local rivals Newcastle 3-0 at St James' Park as they continue their fight against relegation.
Sunderland remain just three points above the drop zone in 15th position, with Wigan holding a game in hand over them, as well as 16th placed Stoke and 17th placed Aston Villa.
Fletcher has helped the Black Cats with 11 Premier League goals this season, last finding the net against QPR in March, though that goal proved ineffective as they lost 3-1 on the day.