Dani Olmo scored an excellent hat-trick to secure a 3-0 win for Leipzig, with Bayern finding it difficult to get going during a lacklustre performance in Munich.
Mathys Tel started at centre-forward for the German champions, with Kane introduced for his debut in the 64th minute, but the England captain found it tough to work his way into the match.
Tuchel said after the match that he was "sorry for Harry Kane", with the Bayern manager struggling to explain why his team found it difficult in the contest with Leipzig.
"I'm just sorry for Harry Kane, he probably thinks that we haven't trained for four weeks," said Tuchel after the clash with the DFB-Pokal winners.
"It's frightening, unexplainable. It was the case last season and clearly we have completely disintegrated to that frequency again today."
Bayern will now switch their attention to their opening match of the 2023-24 Bundesliga campaign, which comes against Werder Bremen on August 18.
The Bundesliga champions completed a move for Kane on Saturday, with the Three Lions skipper moving to Allianz Arena for an initial fee in the region of £86m.
"We're disappointed. We created chances in the first half and then conceded simple goals at the other end. The discrepancy between the form, the mood and what we see out on the pitch is big," Tuchel added.
"The willingness, enjoyment of play, form and condition we played with in the friendly matches has nothing to do with how we played today."
Meanwhile, Bayern goalkeeper Sven Ulreich added: "We all imaged the game going differently. We didn't get into the game, were lacking the desire and physicality from the start. Leipzig did Leipzig things.
"They weren't in the game a lot but countered and were dangerous with every attack. We can't let that happen. We had chances and should've got one back straight after the break.
"Then we conceded a dumb penalty and it's obviously then tough. We have to keep working now, put this game behind us. You get games like this. We have to look ahead. From tomorrow it's full focus on Bremen. We want to win the first three points of the season there."
Kane has become Bayern's fourth signing of the summer transfer window after Kim Min-jae, Konrad Laimer and Raphael Guerreiro.