Song missing for Gunners
The Cameroon international would have been a likely deputy for the injured William Gallas in the centre of defence but has been ruled out after suffering cartilage damage.
Manager Arsene Wenger admitted: "It was a bad surprise. He had a scan on Sunday afternoon on his knee and it is inflamed cartilage. We will have to wait and see how serious it is but he is completely out of the squad for Tuesday night's game."
Thomas Vermaelen looks certain to take one of the central defensive berths as Barca arrive at the Emirates Stadium following a first-leg 2-2 draw in the Nou Camp.
But with Gallas and Song both missing, Wenger may be pressed into an uncomfortable call on veteran Sol Campbell's match fitness, with Mikael Silvestre his only other frontline option.
"That will be a decision I will have to make," Wenger said on Campbell, whose advancing years have led to careful management of his workload. "We have to be conservative with him and see how he feels after Monday."
Captain Cesc Fabregas will miss the rest of the season after suffering a cracked bone in his leg, while Russian Andrey Arshavin is also out after he, like Gallas, sustained a calf injury in the first game.
England forward Theo Walcott is pressing to start after making an impact from the bench last Wednesday and impressing in the weekend win over Wolves, but Robin van Persie (ankle) is still a couple of weeks away from a return.
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