The World Cup-winning defender, who has made only 11 appearances for Arsenal this season, said in an interview with German publication Der Spiegel that his "body was finished" and that he would rather sit on the bench or in the stands.
Arsenal had already announced the 33-year-old would take up a role with the Premier League club's academy next season.
Wenger was asked about Mertesacker's comments at a news conference on the eve of Thursday's Europa League last 16 clash at home to AC Milan which they lead 2-0 from the first leg.
"The quotes weren't a surprise for me," Wenger said.
"To be a pro footballer is high expectations. They are very demanding of themselves and that creates stress.
"There is huge expectation from the environment they are in and that creates stress. To play at the level they play you need a huge resistance to stress. You are depending on your body."
Wenger described Mertesacker, who he signed from Werder Bremen in 2011, as the model professional and said he could change his mind about playing again.
"I think once you stop for a few months you are ready to play again," Wenger said. "Of course, he's had a very long career and he puts pressure (on himself) in every single training (session).
"He never plays one minute without being totally focussed and that demands a huge amount of effort which is why he made the career he has made."
Wenger's side have had an upturn in form after a four-match losing sequence in all competitions, beating Milan away followed by a 3-0 Premier League win at home to Watford.
However, they are 12 points adrift of fourth-placed Liverpool so their most realistic hope of qualifying for next season's Champions League is by winning the Europa League.
Midfielder Aaron Ramsey said finishing off Milan would not be easy but the Europa League was a 'massive opportunity'.
"We saw how good they are and we can't take our foot off the gas," the Welshman said. "We'll treat it as 0-0 and go out to try and win the game, that's the right mentality.
"It's a massive opportunity for us. This game is very important for us and our season."
(Reporting by Martyn Herman; Editing by Ken Ferris)
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