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Watch Barcelona vs Racing Live StreamingThe 23-year-old Frenchman picked up the injury during the 2-1 dfeat to Porto at the Estadio do Dragao on Wednesday.
Arsenal manager Arsene Wenger said he only knew of the injury a day after the match.
He told Arsenal TV Online: "We lost Abou Diaby [at Porto]. It is not understandable. I don't know how it worked. But he has a medial knee ligament injury.
"We discovered it (on Thursday). He came in Friday and he had pain in his knee.
"However Song could be available, we have a test tomorrow on him."
Wenger also summed up his assessment of Arsenal's injury woes ahead of their home clash against Sunderland.
"I don't know about (Manuel) Almunia, we'll have a test 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.
