[citation needed] On 8 February 2017, Retsos was appointed captain of the club in a Greek Cup goalless home draw against Atromitos.

[9][10] On 2 August 2018, Retsos suffered a heavy blow to the quadriceps during pre-season training with Bayer Leverkusen and was diagnosed with tendonitis in the right thigh.

On 29 November 2018, Retsos returned to action in a UEFA Europa League home game against Ludogorets,[11] but suffered a new injury in his left thigh.

[12] On 29 October 2019, after almost a year out with injury, Retsos played in a DFB-Pokal game as a substitute in a 1–0 home win against Paderborn SC.

[13] On 1 February 2020, Retsos signed for Premier League club Sheffield United on loan from Bayer Leverkusen until the end of the 2019–20 season with an option to buy.

[14] On 4 October 2020, Retsos joined Saint-Étienne on loan with a purchase option of €6.5 million in the summer of 2021, becoming an obligation if he makes 20 appearances.