The Gunners have lost four of their last six games in all competitions, falling off the pace in the Premier League and facing a Champions League exit after losing 5-1 to Bayern Munich last month.
The Bundesliga giants travel to the Emirates Stadium for the return leg of their last-16 clash with reports of infighting and general discontent among the Arsenal dressing room and a growing apathy towards Wenger from the fans.
Captain Mertesacker could be involved in his first game of the season having recovered from a serious knee injury and he admits the players have let Wenger down of late.
"I think we saw on a lot of occasions that we were absolutely not ready for a fight," he said.
"That's what I want to see tomorrow, that we are up for a fight, to challenge something
We haven't competed a lot in recent games, we need to turn that around.
"I think it's up to us to show a different face tomorrow
That is really something we are going to demand because we literally blew it away in the second half there (in Munich) and being in that position.
"So it's really down to us now to make the first step to make our fans proud of ourselves again
That is really something that we need the emphasis on."
Amid reports of Alexis Sanchez being involved in a training ground bust-up before being dropped for Saturday's defeat at Liverpool, Mertesacker said things are "tense" but that the players are all behind Wenger.
"The one thing about the Liverpool game is that there were 11 players on the pitch
We had a real chance to go for it, to push it even without our best player on the pitch," he said.
"It's not always the manager
It's hard for him now to select, he does it with his best knowledge and we trust him
He has got a lot of options now
"It is not going to be about one single player and all these questions about one player is very disruptive to our team
Tomorrow we have got a game to face that is really important to our team, not to one individual player.
"We have shown on many occasions that we can turn it around and we are happy to play for him, yes.
"It is a little bit tense but that is normal when you have a stretch like this
But you can feel in every training session that we are eager to change something about the situation.
"I am telling the players that we need to take the first step to get the fans on board again and to be successful again, we need our fans to back us up
"We have done something wrong in recent games, you always do something wrong, but this time in the last couple of games, even more so
We need to analyse very well and then come back with a stronger performance, and learn from the mistakes that we have made recently.
"The spirit is a bit tense, as I said, but that is normal in these circumstances."
Source : PA