'Dumbstruck' Tuchel believes Bayern Munich 'have an obligation to win'
Tuchel, 49, has replaced the sacked Julien Nagelsmann in charge of the six-time Champions League winners.
Nagelsmann lost his job at the German champions on Friday with the Bavarians second in the table but he had guided them to next month's quarter-finals in Europe's top-tier competition.
Tuchel, was fired from Chelsea in September despite guiding them to the 2021 Champions League and spoke to Bayern bosses late on Tuesday.
"Believe me or not, I was a bit dumbstruck in the first 30 seconds of our first discussion," Tuchel told reporters.
"I didn't know what we were talking about. It became clear that it was for right now.
"I was completely surprised. The timing was surprising. There was no contact beforehand," the former Paris Saint-Germain boss added.
Tuchel has a reputation for occasional abrasiveness and demands total backing from his employers - a requirement which strained relations at both Chelsea and PSG.
Bayern's sporting director Hasan Salihamidzic defended the move to get rid of 35-year-old Nagelsmann and denied the decision to hire Tuchel was done in panic.
"The situation between the coach and the team no longer worked," Salihamidzic said.
"We came to the conclusion that we needed a coach like this, who has gained a lot of experience, also abroad.
"On Tuesday evening we met and had a very good conversation. He was very excited from the first second," the former Bayern winger added.
'Obligation to win'
Tuchel, who has signed a deal until 2025, has inherited a side including Senegal attacker Sadio Mane, Germany's Joshua Kimmich as well as Canada wing-back Alphonso Davies.
"The squad is one of the best in Europe currently," he said.
"It's a huge challenge," he added.
The Bavarians are still in the hunt for three titles this season with an eye on the Champions League last eight meeting with Manchester City with the first leg on April 11.
The fixture at City comes seven days before the German Cup quarter-final with Freiburg.
"The DNA is known, there's an obligation for victories," Tuchel said.
"It's an obligation, there can't be a misunderstanding," he added.
Tuchel's first match will come as soon as April 1 after the ongoing international break.
Bayern will meet league leaders Borussia Dortmund, the club that Tuchel led from 2015-2017 before he left for PSG.
"It's the (big) match in German football," he said.
"It has another level of importance," he added.