Striker Tyrone Barnett has returned to Peterborough after suffering a knee injury during a loan spell at Ipswich.
The 27-year-old had been scheduled to stay with the Suffolk club until 19 January but damaged a posterior cruciate ligament during training.
"We're looking at three months before he's back playing," Posh manager Darren Ferguson told BBC Radio Cambridgeshire.
"It's fairly serious. We don't have to operate on it, it'll just be a case of him being put in a brace."
Barnett is Peterborough's record signing, having cost over £1 million when they bought him from Crawley Town at the end of last season.
However, the club were looking to sell him when the transfer window opens next month after a breach of club discipline which led to him being sent out on loan.
Cash for Barnett, who played three games for Ipswich and last month said he would be "delighted" to leave Peterborough on a permanent basis, would have helped Ferguson strengthen his squad.
"It's a bad situation because obviously he's out on loan for a reason. We'd have been happy to sell him and probably would have got good money for him," added Ferguson.
"I know [Ipswich manager] Mick [McCarthy] was fairly happy with him.
"By the time he gets back the transfer window will be shut, but I think we'll carry on and the ones we've identified we'll probably try and still get in."
Meanwhile, Peterborough have agreed to extend the loan of winger Nathaniel Mendez-Laing to Portsmouth for a further month.