Liga, Müller made his competitive debut during the 2015–16 season when he played in the 5–1 defeat to Chemnitzer FC on matchday 32, taking the place of goalkeeper Jannik Huth.

[5] As a result, Müller initially became the first-choice goalkeeper ahead of Benjamin Uphoff and Mark Flekken, following Alexander Schwolow's departure to Hertha BSC.

[6] In October 2015, Müller was called up by the coach of the Germany national under-19 team, Marcus Sorg.

He made his debut on 9 October 2015 during the 1–0 victory in the friendly match against the Mexico national under-19 football team, after coming on at the start of the second half for Timo Königsmann.

He also made additional appearances in friendlies against the under-19 teams of Serbia on 15 November 2015 and South Korea on 29 March 2016, both ending in 1–0 wins.