Wenger's team are now 10 points behind Premier League leaders Chelsea after losing to previously winless Norwich for the first time in 20 years on Saturday.
An error from Arsenal goalkeeper Vito Mannone presented Grant Holt with the goal that condemned the Gunners to their second league defeat of the season and Wenger's grumpy mood after the match reflected his frustration at a lacklustre performance.
Arsenal will need a much improved display to see off German side Schalke and maintain their 100 percent start to the Champions League campaign.
And Wenger hopes the embarrassment of losing to struggling Norwich will provoke a reaction from his players on Wednesday.
"I hope it will have a positive impact," Wenger said. "It was a shock to the system, but you have to give credit to Norwich because they played well, were completely committed and they deserved the win.
"Maybe we underrated the difficulty we would face, but the Premier League is the Premier League - if you are not ready for the fight then you will always have bad surprises.
"There is a big difference between the amount of possession we had and the amount of chances we created, that is down to the fact that Norwich defended well and we did not create a lot."
Wenger had included Jack Wilshere in the squad that travelled to Norwich for the first time in 14 months as he steps up his recovery from ankle and knee problems.
But the England midfielder didn't come on at Carrow Road and is likely to have to settle for a place on the bench at best against Schalke.
Gunners winger Alex Oxlade-Chamberlain, who had to go off with a hip injury against Norwich just 10 minutes after coming on in the second half, is definitely ruled out.
"I don't see anyone new being (back) for Wednesday night," Wenger added. "The closest will be (Kieran) Gibbs, but he will not be ready. (Theo) Walcott will not be ready."
With Gibbs likely to miss out again, Andre Santos will continue at left-back and the Brazilian has warned that Arsenal face a tricky test against Schalke.
The Gunners opened their European campaign with successive victories against Montpellier and Olympiakos to top Group B, while Schalke have never won a competitive tie in England, losing four times and drawing once.
But Santos won't take anything for granted.
"Matches in the Champions League are never easy - you play against world class players and you need to show character and strength at all times," he said.
"Look at our match in Montpellier - we went 1-0 down and we had to fight to get back into the game, which we did very well and won the match.
"We have another difficult match against Schalke coming up, but I believe Arsenal has the quality and I think we will grow game after game in the competition."
Should Arsenal progress, it would be the 13th season they have made it through from the group stages.
Santos knows full well securing qualification with matches to spare would have a positive impact.
"The earlier we qualify in the group stage the better and this will boost our confidence for the knockout stages," he said.
"We are in a very strong position with the good start, but we are not qualified yet.
"We have to play a very strong team in Schalke twice yet, so we have to keep focused - but if we play to our potential, we are confident we can challenge for the (Champions League) trophy this year."