Wenger's hand in the transfer market was forced when Gibbs damaged a thigh in the 2-2 draw against Liverpool on Wednesday night.
Initially it was expected that the left-back would be sidelined for three weeks, but that has now been extended to between four and six.
However, Spain international Monreal was purchased for £8.5 million from Malaga on deadline day and Wenger has hinted the new arrival will come straight into the team, starting with Saturday's game against Stoke.
"Our scouts have followed him for a while and we had a blow with the Gibbs injury," Wenger said. "It will be four to six weeks [out].
"He [Monreal] can play as a left winger too and he can adapt very quickly. Yes [he can improve our defence] but we want him to take part in our collective play."
Monreal is cup-tied for the Champions League so Andre Santos may have to feature against German league leaders Bayern Munich.
The Brazilian has come in for criticism this season from Arsenal supporters and Wenger says Santos has been unfairly targeted at times.
"He is the type of player who gets sharper from competition," Wenger said. "Yes [some criticism he gets is unfair].
"I believe Gibbs will be out for the Champions League. I [cannot] play Monreal. Santos could have a key role."