Arsenal manager Arsene Wenger says his club were obliged to sign Spanish left-back Nacho Monreal due to the injury sustained by Kieran Gibbs.
Monreal arrived from Malaga late on Thursday in a deal reported to be worth Â£8.3 million ($13.2 million, 9.7 million euros).
Gibbs is set to be out for up to two months after limping off with a thigh problem during Arsenal's 2-2 draw at home to Liverpool on Wednesday and Wenger said it made perfect sense to move for Monreal.
"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 on Friday.
"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."
With Andre Santos already on Arsenal's books, Wenger now has three specialist left-backs at his disposal, but Wenger says it is important to have cover in the position.
"I believe we need two left-backs. You cannot play every single game with the same player. We had two before, we have two (fit) again now," he said.
"If Gibbs is out for four to six weeks, from six it can easily go to eight and that means two months in a very decisive period, so it is vital to have another left-back."
Having played for Malaga in the Champions League group phase, Monreal will be cup-tied for Arsenal's last-16 encounter with Bayern Munich, but Wenger feels he can still make a big impact in domestic matches.
"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."