 In the next fixture he received a red card in Turkish Cup against Alanyaspor due to spitting on his opponent and he was banned for an additional 4 matches, attracting interest from clubs like Leeds United.On 31 January 2019, Galatasaray announced the signing of Diagne for a transfer fee of €10 million, signing a contract until 2023.He made his debut for the club on 10 February 2019, in a match against Trabzonspor, scoring the opening goal from penalty in a 3–1 win.