The game of Bayern Munich with Manchester United showed that the club is not perfect with the defense and 2 goals of MU is a vivid illustration of this. Galatasaray, too, under certain circumstances, can rest and achieve results, so you should not discount them and give an unconditional victory in advance. Bayern Munich has certain moments when the team gets lost and allows the opponent to get back into the game again.
I don't think it was just defense that was a problem and Onana also made several mistakes which put the ball into the net. Bayern Munich's performance against Manchester United also didn't look that good, because even though they were two goals ahead in the first half they conceded in the second half. This shows that Bayern Munich's defense was also not very good in that match so this should be a special concern for the coach in the upcoming match.
Facing Galatasaray, Bayern Munich must pay special attention to their defense if they don't want to concede. Galatasaray will probably play quite carefully in this match and playing at home gives Galatasaray the opportunity to get victory or the match ends in a draw.