Diallo joined Rennes and was among a host of internationally recognized youth talents the club had comprised.

These talents included youth internationals Elliot Sorin, Wesley Yamnaine, Jérémy Helan, who played for Manchester City in England, Abdoulaye Doucouré, and Axel Ngando.

Antonetti preferred the 17-year-old ahead of Patrice Luzi and Florent Petit, the third and fourth goalkeeper, respectively.

[7] The following week, Diallo was named the starting goalkeeper for the club's league match against Lyon, almost an hour before the match was set to be played, due to an injury picked up by starting goalkeeper Nicolas Douchez the previous day.

[11] Following his release from Gençlerbirliği, Diallo joined EFL Championship side Nottingham Forest on 14 September 2020.

In May 2018, Diallo was named in the Senegal national team's 23-man squad for the 2018 FIFA World Cup in Russia.