This game against Galatasaray can be compared a bit with the game against Villareal in the knockouts when Bayern Munich got eliminated because they obviously didn't take the game seriously enough. But despite the issues that Bayern displays in some of the game, I am sure that they will not underestimate Galatasaray as this is not some unknown team that nobody knows anything about. I think it is already sufficient information that Galatasaray was able to beat Manchester United at Old Trafford and Bayern Munich had their own problems to beat Manchester United at the Allianz Arena. But in this game I don't think Bayern will score many goals and win the game with only medium effort. Galatasaray will try to push them against the wall and nonstop interfere with their ball possession strategy.
It is impossible for Tuchel to underestimate this match because after all, Galatasaray is a tough opponent for them, so the slightest mistake could lead Bayern Munich to defeat. Bayern Munich will definitely not waste the opportunity to get full points in the group phase and I am quite sure they will play with a core squad to avoid defeat. Galatasaray also has experienced strikers in the team so Bayern Munich must be quite wary of them.
Bayern Munich did not play in their best condition when facing Manchester United because even though they were playing at home to Bayern Munich, Manchester United managed to score three goals and Tuchel had to prepare a solid defense in this match to stop Galatasaray's attacks.