Fletcher had an operation on Monday and now faces a battle to be fit for the start of the new Premier League campaign, with the club yet to reveal details of his recovery period.
The Scotland international, who made the move to the Stadium of Light from Wolves for £12m in the summer, is the club's top scorer this season with 11 goals.
Fletcher has missed the last three matches after suffering ankle ligament damage while on international duty last month.
News of the frontman's absence for the remainder of the season was announced on the same day as skipper Lee Cattermole was ruled out with a persistent knee problem, and represented a major blow to then manager Martin O'Neill with the club fighting for Barclays Premier League survival.
O'Neill revealed at the time that he was hopeful Fletcher would not need surgery, but admitted it was a possibility if the ankle did not settle down with treatment and rest.
The Ulsterman was replaced by Paolo Di Canio on March 31 with the newcomer well aware he would be without the man most likely to fire the Black Cats out of trouble.
In the circumstances, he challenged the rest of his players to come up with the goals to keep them in the top flight, and received the perfect response on Sunday when Stephane Sessegnon, Adam Johnson and David Vaughan struck to secure a famous 3-0 derby victory at Newcastle.
However, Sunderland remain only three points clear of the relegation zone with five games to play despite Wigan's 1-0 defeat at Manchester City last night, and the Latics still have a game in hand on their fellow relegation contenders.
The Black Cats entertain high-flying Everton at the Stadium of Light on Saturday, when anything other than three points could dissipate the euphoria of their heroics on Tyneside.