The 30-year-old former Everton and Hull City striker joined as a free agent on March 26 but injured his groin in the build-up towards a first training game to try to prove his fitness.
He went for a scan on Tuesday, and the results suggest he will not play again this season. His short-term contract expires at the end of the campaign.
“The scan was on Tuesday and the result was not positive,” said coach Carlos Corberan “He will be around eight weeks to recover so we don't expect to have him from now until the end of the season.
“In the preparation he was doing for the training game we were using to make him ready for the team he suffered an injury and we need to just keep going with the options right now, and to help him to have a good, fast recovery because he's part of our squad even if he cannot play the games.”
Asked about Niasse's future with the club, Corberan said: “What happens after this season I'm not focused on because I'm just focused on making the team perform well tomorrow.”
There was some positive news, though, with 25 players due to train on Friday, giving the likes of Ryan Schofield, Pipa and Alex Vallejo a chance to prove their fitness for Saturday's vital game at home to Rotherham United.
Schofield missed Tuesday's 7-0 defeat at Norwich City through illness.
“We are going to check how he is in training and if everything goes well, he will be the keeper on Saturday,” said Corberan of Schofield. The same applied to Pipa, Vallejo and Josh Koroma and Rolando Aarons, who have been out for longer periods.
Centre-forward Fraizer Campbell was on the bench at Carrow Road, but Corberan explained he should be available from the start against the Millers.
“After the Brentford game he felt a problem in his Achilles that he has been trying to manage,” he said. “Fortunately now we have other options because for so many games this season he was our only option as a striker, but now we have (Yaya) Sanogo and Danny Ward.
“Maybe because of that he's suffering with his Achilles and that's why the medical advice was to not involve in many minutes in the previous because he wasn't in 100 per cent condition for the game.”
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