Thought Koo was the best man on the field and Malli the worst. Who gets substituted first? Mainz visibly fell apart after that. Without Park they then lost the previously stable focus on the left side. They really need a faster defender btw., Noveski's lack of speed got regularly exposed in both matches.
they had that issue since last season but Tuchel covered it up by playing 4-3-1-2/4-4-2 diamond imo (or other tactical switches). anyways, it was Koo but i believe Koo and PJH were subbed off near each other.
Koo got subbed off at 69, Park at 74. Mainz is really lacking chemistry being so early in the season, but the left side (Okazaki-Malli-Koo-Park) really worked well compared to the rest (which also helped keeping the opponent busy) so it's odd that being deconstructed first in both matches. Not sure what Hjulmand wanted to achieve there, Tuchel certainly did better at covering for the defense tactically. Jara and Diaz, who came on for Koo and Park, could be seen as defensive changes, but the effect was the opposite.