Arsene Wenger has described Santi Cazorla's ankle injury as the worst he has seen, but remains hopeful the Spanish midfielder will return to action in January.
Cazorla, 32, has not played for Arsenal since October 2016 and has undergone eight operations to address a longstanding Achilles problem.
He revealed earlier this month a gangrene infection had "eaten" eight centimetres of his Achilles tendon and said he had been told by doctors he should be "satisfied" just to be able to walk again.
"It is the worst injury (I have known)," Wenger told his weekly press conference at Arsenal's London Colney training centre on Thursday.
"It started with a pain in the Achilles and it has gone from worse to worse. "I know how much Santi loves to play football and loves to be out there every day and I feel very sorry for what happened to him, because it is unbelievable.
"I saw him one month ago, he was here, and I told him to prepare as well as he can. He has competent medical people around him and he tries to practise. "I have not spoken to him recently, but I hope he comes back. I left him out of the squad, hoping he would be available in January."
When it was suggested Cazorla might never be able to play again, Wenger replied: "Let's not talk about that, but I hope you are wrong."
Spain international Cazorla, who joined Arsenal from Malaga in 2012, recently told Spanish newspaper Marca he aims to be back playing in January.
Wenger confirmed Olivier Giroud will miss Saturday's north London derby at home to Tottenham Hotspur after the France striker injured himself on international duty.
It means Alexandre Lacazette is likely to start up front, having been left on the bench for previous high-profile games against Liverpool and Manchester City.
Lacazette, who scored twice in France's 2-2 friendly draw with Germany, has completed 90 minutes only once since his close-season move from Lyon.
Centre-back Shkodran Mustafi (thigh) and forward Danny Welbeck (hamstring) are due to undergo fitness tests to determine their availability.