Peterborough defender Kgosi Ntlhe has undergone toe surgery and looks set to be out for two months.
The 21-year-old had been back in training having been out since the game against Sheffield United on 18 August.
But he has needed to have an operation on his ligaments to solve the problem.
"He trained with us last week with the physio, he was running and doing everything he was asked of him, but had a setback," Posh caretaker boss Grant McCann told BBC Radio Cambridgeshire.
Ntlhe's injury means McCann will look to extend the loan spell of Leicester City defender Callum Elder, who joined on a one-month loan deal on 29 August.
"I've spoken to Callum and (director of football) Barry Fry and I'll speak to the chairman about it, but he's definitely one I'd like to keep here," said McCann.
"He's a good player, the lads like him and he's a great player to have around the place."