The Gunners moved to push through a deadline day deal, believed to be for £8.5million, after England defender Gibbs hobbled off in Wednesday night's Barclays Premier League 2-2 draw against Liverpool at Emirates Stadium with a thigh problem for which the left-back could be ruled out of action for possibly the best part of two months.
"Our scouts have followed his progress for a while and of course we had a blow with the Gibbs injury, which is a bit longer than expected," Wenger said ahead of Saturday's match against Stoke. "We expected two to three weeks, now it will be four to six, so we were in a position where we had to make a quick decision. That explains our buy."
Wenger continued: "I believe we need two left-backs. You cannot play every single game with the same player. We had two before, we have two again now.
"If Gibbs is out for four to six weeks, from six it can easily go to eight [before he is competitive again] and that means two months in a very decisive period, so it is vital to have another left back."
Monreal, 26, will be cup-tied for the Champions League having helped Malaga qualify for the knockout stages.
However, Wenger is in no doubt the Spaniard can have a big impact within the Gunners squad.
"We believe he can integrate our style of play and adapt very quickly to that," said Wenger.
"He has the characteristics to play our game because he is very mobile, has a good left foot and is very good in combination play. It is of course vital in our style of play."
Wenger has yet to decide whether Monreal will go straight into the side to face Stoke, but revealed midfielder Mikel Arteta could be back from his lay-off with a calf injury. Centre-back Thomas Vermaelen aggravated an ankle problem against Liverpool and will be assessed.
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