Mertesacker has left Sanchez alone since the end of the season to make up his mind on his future — and quotes emerging on Saturday night don’t look good for the Gunners.
Manchester City are leading the chase for Sanchez, who has only 12 months left on his current deal and has been involved in a long-running stand-off.
World Cup winner Mertesacker said: “He needs a break now. He needs to relax. He has done a lot for us. He played almost every game last season and then the Confederations Cup.
“You need to leave him alone to make up his mind what he wants to do. It’s his decision. If he feels good, comfortable, he can play with freedom. He has his qualities. You cannot find those everywhere.
“He has shown them and we have embraced him.”
Mertesacker enjoyed a fairy-tale finish to last season as Arsenal’s man-of-the-match in the FA Cup Final and will carry on for one more year before heading up the club’s academy .
Despite his future being mapped out, Mertesacker is determined to make the most of his final season as a player.
The German barely played last season because of a knee injury, which is still being managed and is aware of the “captain’s curse”, which has seen every player given the armband – since Patrick Vieira left in 2005 – suffer a long-term injury.
Mertesacker admitted he was aware of it, adding: “It doesn’t matter to me. I have lots to give. As long as I am there, I want to play and contribute.
“Being able to train and make an impact makes me feel good. And pushing the other players is something I like because, when you are injured, you can do nothing, which was horrible for me.
“Last season was not satisfying for me. The end was a fairy tale, but the way to that was very tough. I want to stay injury-free, be an option for the manager, get picked every weekend. That’s the target.
“They manage me very well. I am not doing everything because nobody knows how my knee reacts. We are not overloading. I am fine, but I am not doing every double session. When we come to games, you will see the reaction.
“At the end of the season, that was OK, but I need to really find out how well I am when it comes to playing 90-minute games.”
Mertesacker also has high hopes for Arsenal’s defensive protege Rob Holding, who has been given an extended break after the Euro Under-21s.
Mertesacker added: “It’s very important Rob sticks to what he has shown. Not being selected at the start, but still managing to do extra sessions.
“He has a great character. If he does the same work with the same habits, there is lots to come. I am trying to help him as much as I can for the next step, playing regularly.
“Being out does not help, because of the U-21 tournament, but it’s really important for him to recover.”