 On 2 August 2018, Retsos suffered a heavy blow to the quadriceps during pre-season training with Bayer Leverkusen and according to the initial medical evaluation, he was diagnosed with tendonitis in the right thigh.The injury is expected to sideline the defender for at least six weeks.It is Retsos’ first serious injury with the German club and he will miss the kick-off of the 2018–19 Bundesliga season.