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.
With the prognosis of that injury now more concerning - set to rule the left-back out of action for possibly the best part of two months - Wenger was glad to have been able to conclude a deal for a player he had been tracking for some time.
"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 at Arsenal's media conference 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 on Saturday, 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 on Friday.
Arsenal were linked with several other players during the January transfer window, including Barcelona forward David Villa and West Ham's Senegal midfielder Mohamed Diame.
Wenger admitted there was some frustration he was not able to get at least one more body in.
"We were close to one more signature, yes," the Gunners boss said.
"It was not a money problem, it was more a desire from other clubs to keep their players."
Despite not being able to further bolster the group, Wenger has every confidence they can finally deliver consistency over the final four months of what has been a stop-start campaign.
"I believe that we have top quality in our squad and that is what we want to show until the end of the season," he said.
"The game against Liverpool shows the quality we have, but we also have some aspects in our game that we need to improve.
"How active we will be in the market in the summer will be decided how well we do now until the end of the season."
Relegation-battlers QPR and Newcastle were the big spenders of the January window.
Wenger has always been unimpressed by the whole system.
"To make it as simple as possible, the teams at the top didn't buy at all, and the teams at the bottom did buy," he said.
"[That is] explainable by the fact that to strengthen the teams at the top in the middle of the season is normally impossible, and by the fact that teams today are scared to go out of the Premier League, [that they] are ready to do anything in winter not to go down."
Wenger, though, is not expecting an easy ride against Tony Pulis' men, who the Gunners boss believes have again raised their game.
"When you play Stoke, you always expect a fierce resistance, a very committed game and unfortunately, you are never disappointed. You always get what you expect," he said.
"I believe they have improved their technical quality as well, with players like Steven Nzonzi and Charlie Adam, therefore they have more variety in their game than they had in previous years."