Arsene Wenger has revealed that Abou Diaby sustained knee ligament damage against Porto on Wednesday.
The French midfielder picked up the knock during Arsenal's disappointing 2-1 defeat at Estadio do Dragao.
The Gunners are yet to determine the full severity of the problem, but accept that the 23-year-old could be set for a lengthy spell on the sidelines.
Diaby's setback has been offset by positive news regarding Alex Song's fitness, but Wenger is still expecting to be short on numbers for Saturday's home date with Sunderland.
He told Arsenal TV Online: "We lost Abou Diaby last night [at Porto]. It is not understandable. I don't know how it worked. But he has a medial knee ligament injury.
"We discovered it today (Thursday). He came in today and he had pain in his knee.
"However Song could be available, we have a test tomorrow on him.
"I don't know about (Manuel) Almunia, we'll have a test tomorrow on him too. I'd say Song is 80 per cent to be back in the squad, Almunia is 50/50, but Diaby is zero.
"(Andrey) Arshavin, (William) Gallas and Eduardo will still be out."
Should Almunia fail to shake off an injury to his hand, Lukasz Fabianksi is likely to provide cover once more between the sticks.
The Pole endured a torrid night in Portugal in midweek, putting through his own net and conceding the free-kick from which Porto grabbed their second, and there was a fear his confidence could be shattered.
However, his team-mates retain complete faith in his ability, with club captain Cesc Fabregas insisting Fabianski is a more than competent shot-stopper.
"He is a very good keeper," said the Spaniard.
"He had his chance and sometimes it doesn't go your way.
"But we will stand up and support him until the end because he is a good character and I am sure he will be back."
Fabregas added: "Lukasz is strong and we have to be there for him. Sometimes when you are young in a big game, these things happen to you."