The Bavarians have struggled in the past weeks since Tuchel's arrival in late March, having been eliminated in the German Cup last eight by Freiburg before a 3-0 loss at City in the first leg last week.
They then slumped to a 1-1 home draw with Hoffenheim in the Bundesliga on Saturday to pile more pressure on the Bavarians, who also banned forward Sadio Mane for a game after he hit teammate Leroy Sane after the game in Manchester.
Tuchel said any talk of a 4-0 or 5-0 win against City, a team that is unbeaten in their last 14 matches in all competitions, was fanciful.
"We cannot talk about a miracle and expect it to just come by talking about it. We have to have a realistic level," Tuchel told a news conference on Tuesday.
"To talk about 4-0 or 5-0 is not appropriate and given our performance on Saturday I don't know if everyone believes it 100 percent."
"What we want to do is win the first half. That is the first goal. Then anything can happen, if we also get that bit of luck which we did not have in the first game. It is a huge mountain to climb. We want to believe but don't want to be dreamers."
Bayern are struggling for goals, having struck only three times in their last four matches in all competitions, and with a 3-0 deficit to make up they need to step up and deliver on Wednesday.
"We are not happy and we are working on it," said Tuchel of his team's finishing. "We are missing some confidence, some lightness. You can feel it. We struggle with it. We overthink it."
Bayern will have forward Eric Maxim Choupo-Moting back from injury to boost their options in attack with the Cameroon international their top scorer across all competitions this season.
"We are responsible to do everything to deliver a top performance because otherwise nothing is possible," Tuchel said. "We will take it step by step, half by half, our first step is to win the first half."
"We are still in this competition," Tuchel said when asked about possibly missing out on more silverware this season should they be knocked out. "We can talk about it tomorrow evening."
