Arsenal defender William Gallas

Arsenal defender William Gallas could be on the sidelines for another month after suffering a recurrence of his calf injury.

The France international has not played since the 1-0 victory over Liverpool at the Emirates on February 10 and could now miss another significant chunk of the remainder of the campaign after breaking down in training.

Gunners boss Arsene Wenger admitted the centre-back's problem had gone back to square one and was unsure when the former Chelsea man would be fit.

Sol Campbell has been passed fit for Tuesday night's meeting with Porto but Wenger has admitted he may have to rotate the former England defender with Mikael Silvestre on a match-by-match basis.

Further disruption would be caused by Silvestre's preference to play on the left of any pairing, meaning that the left-footed Thomas Vermaelen would be required to shift to the right.

"Gallas has been out for a month and now he has had a recurrence of his calf injury," Wenger said.

"He felt very well on Sunday and went out but he has gone backwards again. We are at the starting point with Gallas unfortunately.

"At the beginning, it was a minor injury. We don't know how long it will be but for Hull, for example, he has no chance.

"We were impatient with him, we pushed him and he wanted to get on as well; that is why, maybe, it has happened.

"But there is no obvious sign as to why it keeps coming back. He has no pain at all, then he goes for a little jog and it goes again."