Arsene Wenger's side travelled to Portugal without a string of first-teamers, with Andrei Arshavin, Eduardo, William Gallas, Alex Song and Manuel Almunia joining an injury list that already included Dutch forward Robin van Persie.
And having lost in controversial circumstances at the Estadio do Dragao - with Porto scoring their winner from a quickly-taken free-kick - the Gunners have now learned that lung-busting middle-man Diaby is set for spell on the sidelines.
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"We lost Abou Diaby last night [at Porto]," Wenger told the club's official website.
"It is not understandable. I don't know how it worked. But he has a medial knee ligament injury.
"We discovered it today. He came in today and he had pain in his knee."
However, the Emirates Stadium supremo did receive some good news in that both Almunia and Song should be fit in time for the club's Premier League clash against Sunderland this weekend.
"Song could be available, we have a test tomorrow on him," he added.
"I don't know about Almunia, we'll have a test tomorrow on him too. I'd say Song is 80 percent to be back in the squad. Almunia 50/50. But Diaby is zero.
"Arshavin, Gallas and Eduardo will still be out."
