The Germany international has been preferred to Koscielny as Thomas Vermaelen's partner in central defence, but the Frenchman is pleased for his teammate.
"I think everybody sees he is better this season," he told Arsenal's official website. "The English league is very difficult, very strong and different players have a smaller time or a longer time to adapt to this football.
"I am happy for Per because he's a nice guy, very important in the dressing room. He's a joker so he's good for the team. I'm happy because he's played well this season and it's good for the team.
"We have a good team with every position, we have very good players. Now I cross my fingers to not be injured, like last year. We have three or four centre-backs who can play so it's good for the team. The boss (Arsene Wenger) chooses the two centre-backs who play."
Koscielny has started just twice in the Premier League so far this campaign.