Captain Cesc Fabregas did not train with the rest of the Arsenal first team ahead of the Champions League clash with Porto on Tuesday.
The midfielder is battling a hamstring problem in his right leg and limped off after scoring the opening goal in the 3-1 Barclays Premier League victory over Burnley on Saturday.
The tightness was in the same hamstring he injured earlier in the season, which kept him out of action at the turn of the year.
Veteran defender Sol Campbell did join in training and he looks set to recover from a groin problem in time to face the Portuguese side, who will take a 2-1 lead to the Emirates Stadium for the second leg of the last-16 tie.
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