The Scot was stretchered off whilst on international duty, after falling awkwardly on his ankle in an aerial challenge with Wales’ Ben Davies.
Scans have revealed that the hit-man has suffered ligament damage, but further investigation is needed to ascertain the full amount of time he will miss.
At best, he is likely to miss around a month, but he could be sidelined for all eight of Sunderland’s remaining fixtures.
The news will come as a real blow to manager Martin O’Neill, with Fletcher the club’s current top goalscorer.
The 25-year-old has hit 11 Premier League goals so far this season, exactly a third of the Black Cats’ total.
With back-up striker Danny Graham yet to find the back of the net in the league since his arrival at the Stadium of Light, the club have real attacking problems.
Just four points above the drop zone, Sunderland could find themselves dragged into the bottom three sooner rather than later, with Manchester United, Chelsea and Newcastle their next three opponents.
Sunderland are winless in their last seven Premier League fixtures, picking up three points along the way.
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