Koscielny's stock rose during his second season in English football and he believes Mertesacker is going through a similar transformation.
"I think everybody sees he is better this season," Koscielny told Arsenal Player. "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 chooses the two centre backs who play.
"For me it's important to have a good relationship with my team-mates and when I play with Thomas or Per or Johan, I give myself for the squad. If I'm on the bench, I motivate my team-mates to do well for the squad and winning the game."