Poirier started to play basketball in May 2010 at the Bussy Basket Club. In the 2013–14 season, he finished as the top rebounder of the Espoirs championship, averaging 12 points and 10.8 rebounds in 27 minutes over 30 games.
On 24 April 2014, he signed a three-year deal with Paris-Levallois, but was loaned to Hyères-Toulon of the Pro B. He started the season being a solid rotation player with Davante Gardner. On 14 February 2015, he contributed against Angers by finishing the game with 10 points and 4 rebounds in just 10 minutes.
For the 2015–16 season, he returned to Paris-Levallois. From December onward, he was subject to a special designation allowing him to be able to play both with Paris-Levallois, and with CFBB in National 1. In December 2015, Frédéric Fauthoux replaced Antoine Rigaudeau as coach, which allowed Poirier a greater chance to show his potential. In April 2016, he was named MVP of the Round after finishing the match against BCM Gravelines-Dunkerque with 23 points, 12 rebounds and 2 assists in 25 minutes, when he started the game as a backup.
In July 2016, he participated in the NBA Summer League with Orlando Magic. In his first game, he finished with 6 points, 5 rebounds and 1 assist in 15 minutes. Over five games, he averaged 4.2 points, 4.4 rebounds, 0.6 assists and 0.4 blocks in 11.5 minutes per game. On 11 September 2016, as part of the pre-season, he and his team won the Sarthe/Pays de la Loire trophy in a three-team tournament by beating Le Mans and Antwerp.