The Gunners take on German Bundesliga champions Borussia Dortmund at Signal Iduna Park and Koscielny is feeling much calmer now that ex-Werder Bremen centre-half Mertesacker is in the side after a solid debut in the 1-0 league win over Swansea City on Saturday.
"He is a very good defender with a lot of experience," Koscielny said. "He is a German international, and is really important for us on the pitch.
"He is tall, which is good for our physical side. He will help me on the pitch with defensive corners.
"With Thomas [Vermaelen] it is very different. We must defend very well, and make a clean sheet more possible."