The Gunners splashed around £8.5m on the 26-year-old from Pamplona on transfer deadline day.
"Our scouts have followed his progress for a while and of course we had a blow with the [Kieran] Gibbs injury, which is a bit longer than expected," Wenger told reporters.
"We expected [him to be out for] two to three weeks, but 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 said that Monreal can also play in midfield and believes that he will fit seamlessly into his Arsenal side.
"We bought Monreal as a left-back but he can play as well as a left-sided midfielder," he added. "We bought him as well because we believe he can integrate into our style of play and adapt very quickly to that.
"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. That is vital in our style of play."
Monreal is likely to start today's game against Stoke City at the Emirates Stadium.