Scotland international Fletcher has been plagued by fitness issues for much of the campaign, scoring just three goals in 13 Premier League starts due to a string of injuries.
The former Wolves man has missed the last four games with damaged ankle ligaments, and Poyet conceded that the 27-year-old is unlikely to return before the end of the campaign.
"I don't think Steven Fletcher's going to be available this season," the Uruguayan said.
"We'd hoped he might be able to make the last few games but I'm changing my opinion on that now."
Sunderland approach Saturday's visit of Everton bottom of the Premier League, some seven points adrift of safety with as many matches left to play.
Even though they have two games in hand over the teams above them, Poyet admitted after Monday's 5-1 humbling at the hands of Tottenham that his side require a "miracle" to remain in the top flight.
But the former Brighton boss insists he will not give up hope of survival.
"I am still here until the last minute believing it is going to be a possibility," he added.
"It is a big challenge, it is really, really difficult - people were saying even impossible - so we will see."
