it makes sense if Tuchel tries to replace his goalkeeper with another more experienced player like De Gea but to be honest I actually dont see a big problem with Sven Ulreich as far as guarding Bayern Munich goal, hes not that careful, he just sometimes experiences carelessness and this is normal because all players will experiencing the same thing.
from several cases, I think a little that the reason Tuchel did not glance at De Gea seems to be because Tuchel thought that De Gea would not renew his contract at Man United because his performance had declined, so Tuchel doubted this and chose Sven Ulreich as his mainstay goalkeeper.
You are right that it is normal for players to make mistakes but it will still endanger the team when it occurs in important matches such as the knockout rounds and mistakes should almost never occur which will result in losses for the team, so far Bayern Munich's goal with Sven Ulriech can be said to be good Just because they only conceded 6 goals and of course that's not just because of the goalkeeper's role because defenders have an important role as well as protecting the goal from opponent attacks.
Of course Tuchel has the right reasons why he didn't choose DeGea, if I'm not mistaken one of them, is because he wants a high salary, of course his status without a club makes any club reluctant to make him a player, including Bayern Munich, and it's possible that the former Manchester United goalkeeper's playing style is not suitable with what Tuchel is looking for so he never considered him to be at Bayern Munich.