In the summer of 2015, Diallo was promoted to the senior team at Metz. He played 17 games in his debut season with the club, scoring 9 times.[citation needed] He appeared in 19 matches for Metz during the 2016–17 Ligue 1 season, scoring once. In January 2017, he was sent out on loan to Brest for the remainder of the season after falling out of favour with new manager Frédéric Hantz.[citation needed] Diallo had a successful time in Brittany, playing 16 times and scoring 7 goals.[citation needed] He went back on loan to Brest in the summer 2017 under the management of Jean-Marc Furlan. He spent the entire season there as Brest finished 5th and he scored 9 goals in 33 games in Ligue 2.[citation needed]
After returning from his loan, Diallo had a prolific season with Metz, winning the Ligue 2 title and scoring 26 league goals in 37 games in the process, only bettered by Gaëtan Charbonnier who played for Diallo's old club Brest, scoring 27 times.[4] Diallo was included in the year-end UNFP Ligue 2 Team of the Season squad alongside 6 of his Metz teammates.[5]
Diallo immediately got up to life in Ligue 1, scoring 6 times in his first 8 matches. In the January window he was subject to interest and a reported bid from Premier League club Chelsea, but no move materialised.[6] By the time the Ligue 1 was suspended in March 2020 due to the coronavirus pandemic, Diallo had contributed 12 goals with Metz sitting in 15th position in the table.[7]
Strasbourg
On 5 October 2020, Diallo joined Strasbourg on a five-year contract.[8]