 On 17 September 2017, he made his debut in Bundesliga in a 4–0 home win game against SC Freiburg as a starter.On the club's Twitter account, Retsos was voted the Man of the Match.On 9 April 2018, substitute Retsos scored his first goal in Bundesliga by firing into the far corner after a left-wing free-kick broke to him at the far post, in a 4–1 away win against RB Leipzig.