Arsenal boss Arsene Wenger admits getting his centre-back pairing right has been difficult.

Per Mertesacker and Laurent Koscielny secured a clean sheet in victory at Bayern Munich although Arsenal still crashed out on away goals after their 2-0 win.

Wenger said: "It's true, they do well together but I can't see any obvious reason why Per and Vermaelen should not do well together.

"In some big games, I have also played Vermaelen and Koscielny together. You think that should work as well because they are two good football players, one is right-footed and one is left-footed, they complement each other well.

"However, sometimes there are mysteries in our game that are difficult to rationalise.

"Koscielny has shown a bit more authority than usual and when he gets into that frame of mind, you can see he has outstanding qualities.

"It is something I know he has but he needs that authority in his game to get his qualities completely out."