Pressing and build-up were inefficient once again
There are a few factors that could explain Borussia Dortmund's struggles in this game, even with the starting XI on the pitch. For instance, maintaining possession is one of Sahin's mantras as BVB head coach, which is extremely necessary to keep the game under control and avoid giving your opponent chances. But, in order for that strategy to work, two main structural components need to be on point: pressing scheme and build-up.
As has been a current trend this season, Dortmund’s shaky press scheme made things a lot easier for Mainz, as the Die Nullfünfer effortlessly evaded their high lines. By creating advantages in certain parts of the pitch, they quickly got an extra man who could find an escape route from Dortmund's press. Whether the spaces to progress the ball were found in the flanks or the midfield, the home side always seemed to have the upper hand.
In terms of build-up, Dortmund worked within two different variations. Either a single pivot with Felix Nmecha as the six or Brandt drifting behind to help his teammate in a double pivot. Sahin also instructed his players to try to keep possession as much as possible, even if that meant attracting Mainz's press. There is nothing wrong with that approach, especially if it means playing in transition to hurt your opponent short-handedly.
However, Mainz 05 was not pressing high constantly. Initially, they defended in a 4-4-2 or 5-4-1 in a low block and pressed high only when Dortmund started the build-up from goal kick position. By doing so, Mainz did not allow space for BVB to attack and planned on capitalising on their mistakes.
The poor build-up facilitated their pressing strategy. BVB's set-up was precarious and slow-paced, and the players were too far apart from each other. With few alternatives to build from the back, Dortmund gave the ball away more than Sahin likely anticipated, as Mainz won the out-of-possession battle.