"We were a little impatient with Gallas and we pushed him and he wanted to get on as well, which 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 goes for a little jog and then it goes again.
"Gallas has been out for a month and now he has had a recurrence of his calf injury.
"He felt very well on Sunday and went out - but he has gone backwards again.
"At the beginning it was a minor injury. We don't know how long it will be, he needs a scan and we will assess it from there."
Skipper Fabregas underwent a scan yesterday on the hamstring he tweaked in Saturday's 3-1 win over Burnley and was told there is no major damage. He could even be back at the weekend against Hull.
The midfielder said: "I wanted to play but the doctors recommended I rest and avoid any risk.
"I am sad to miss out because Porto is important to us. I will suffer far more than usual, having to watch from the side."
Wenger said: "There is no surgery needed.
"In the end, I didn't want to take the gamble. To play again could mean significant damage.
"Ideally you want him there, we want to qualify. But the absence of Cesc is not any excuse as we have enough players and a strong enough squad."
Read more: *www.thesun.co.uk/sol/homepage/sport/football/2884439/Wenger-I-crocked-up.html#ixzz0efzRrg0T