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Gunners chiefs have taken up an option in Mertesacker’s current deal to keep him at the club until the summer of 2018.
The World Cup winner has not featured this season due to injury but is closing in on a return to full fitness.
“He has an option that we have taken,” the Arsenal boss. “There was no negotiation, just an option that we have taken.
“He’s back in training, not with the squad, but he’s two weeks away now.”
Shkodran Mustafi and Laurent Koscielny have formed a promising partnership at the back but Wenger is delighted to soon be able to call on Mertesacker, 32, for the business end of the season.
He added: “With the number of games we have it’s important that we have all our experienced players back and that everybody is focused to do well.
“We’re going through a moment of truth now, January until May is five months to go. You want ideally all your players with big experience back in your squad.”
Meanwhile, Wenger has confirmed that Carl Jenkinson is deciding whether to join Crystal Palace while he’s ruled out a move for West Ham rebel Dimitri Payet.