Team-mate Koscielny has impressed supporters and media alike this year with his performances this season, and, given time, Wenger says Mertesacker can emulate his French counterpart.
"He had not completely recovered [from the internationals]," Wenger told arsenal.com. "It is a different game in England, a very demanding game for the defender.
"In every second of the game you are really tested with full commitment, and it takes some time to adapt.
"We have seen that with [Laurent] Koscielny, who played at right-back and was exceptional.
"Last year he would not have delivered this kind of performance because you learn that from fighting in every game. That is why it is so difficult to buy defenders from abroad."