The Cameroon international was unable to train on Monday morning after damaging cartilage during the Gunners' last-gasp 1-0 win against Wolverhampton Wanderers on Saturday.
It comes as another huge blow to Arsene Wenger, who was already without central defenders William Gallasand Johan Djourou.
Blow: Song (being tackled) injured his groin against Wolves
ARSENAL SQUADTeam (from): Almunia, Sagna, Vermaelen, Campbell, Clichy, Denilson, Diaby, Nasri, Rosicky, Walcott, Bendtner, Fabianski, Eduardo, Merida, Eastmond, Traore, Eboue, Silvestre.
The Arsenal boss said: 'It was a bad surprise this morning (Monday). He had a scan yesterday 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 tomorrow's game.'
The Arsenal boss has fears about playing veteran Sol Campbelltwice in four days.
Wenger left Campbell on the bench when Gallas was injured during the first leg of the Champions League quarter-final rather than expose the 35-year-old to Barca's speed and mobility.
Enlarge Dilemma: Wenger leads Arsenal training before flying to Barcelona
'That will be a decision I will have to make,' Wenger said on Campbell, whoseadvancing years have led to careful management of his workload. 'Wehave to be conservative with him and see how he feels after today.'
But there are now few options left as the European champions carry theadvantage of away goals from 2-2 draw at the Emirates. FormerManchester United defender Mikael Silvestre could partner ThomasVermaelen, but Wenger is also mulling moving Bacary Sagna in from right back.
In the running: Campbell (left) could start after training with Nicklas Bendtner
Craig Eastmond, the 19-year-old who can play in defence or midfield, is expected to be part of Arsenal's squad, as is fellow rookie Fran Merida.
Wenger is also without Cesc Fabregas, Andrey Arshavin and Robin van Persie, though Barca are also without central defenders Carles Puyol and Gerard Pique because of suspension.
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