The Scotland international has not suffered any broken bones, but his ligaments have been damaged.
It has been a miserable campaign for Fletcher who has netted just three goals and been plagued by ankle, shoulder and Achilles injuries
However, Gus Poyet expects the 26-year-old to be back before the end of the campaign.
Gus Poyet: “We’re still assessing him.
“Next week, we’re going to have a final decision on him.
“He’s not fit for this weekend.
“It was quite swollen, so it was difficult to do all the tests.
“It wasn’t very clear. We’re trying to stay calm and see what happens.”
"He could miss the remainder of the season, but I hope not.
“But it depends on how badly the twist is.
“He twisted his ankle and there was a fear that there was some bone issues. There aren’t, that’s for sure.
“But there is some ligament damage. We’ll see next week.
“I hope he finishes the season well, keeps himself fit and starts next season right.
“It’s been difficult for him. When I got here, he had the shoulder problem, then had a recovery and played a few games in pain.
“But it’s probably a season to forget for him, because it’s been quite difficult.”