Ouss Cisse is set to be out until next year after having knee surgery.
The 26-year-old midfielder hasn't played since the 4-4 draw with Oldham on October 21, but has undergone an operation to fix a knee problem he was suffering.
While he is expected to return to training in the next few weeks, manager Robbie Neilson said he doesn't expect the Mali international to play again until January.
"He had a problem with his knee which flared up a few weeks ago, so he had a little operation on it," Neilson confirmed on Thursday. "It'll be a few weeks before he'll be back training, so I don't expect to see him before the new year."
Meanwhile, Osman Sow has the protective boot around his injured foot removed today (Thursday) which will allow him to begin running again.
Sow hobbled out of the 1-1 draw with Walsall on October 17, and could be back in the squad before the end of December.
Neilson added: "Osman's boot comes off today, so he'll be out running again. It'll be another couple of weeks before he's back training with the first team squad but he's progressing well.
"We've been unlucky with injuries this season. Peter Pawlett being out, Osman out, Ouss out. Bringing Peter back in was huge because he gives us energy. He was a real coup to get him."