Arsene Wenger expects Nacho Monreal to be a tasty addition to Arsenal's squad after the Spaniard's deadline day move.
The Gunners paid £8.5million for the Malaga left-back, 26, after Kieran Gibbs picked up a thigh injury during the 2-2 draw against Liverpool.
Gibbs could be sidelined for up to two months and Wenger was delighted to bag long-term target Monreal.
The Frenchman said: "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.
"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."
He added: "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.
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"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 will be cup-tied for the Champions League having helped Malaga qualify for the knockout stages.
Wenger is sure the Spaniard can have a big impact on the Premier League.
He said: "We believe he can integrate 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. It is of course vital in our style of play."