Dassy received his youth training at KV Mechelen. At the age of sixteen he could count on interest from AFC Ajax, but the defender remained loyal to his club. In June 2019, the club let him sign a contract until 2021 with an option for an extra season. In May 2021, he signed a contract with the club until 2025. In the preparation for the 2021/22 season, Dassy seemed to be moving up the hierarchy, but he suffered a meniscus injury at just the wrong time.
In the summer of 2022 he seemed to be on his way to Lommel SK, but he eventually signed a one-year rental contract with Helmond Sport, the Dutch sister club of KV Mechelen, at the end of August 2022. On September 16, 2022, he made his professional debut in the shirt of Helmond Sport: on the seventh competition day, coach Sven Swinnen had him come on as a substitute for Peter van Ooijen in the 90th minute in the 2-1 win against FC Dordrecht. Dassy ultimately played six official matches for Helmond Sport, only one of which was as a starter. On November 28, 2022, the club announced that Dassy returned to KV Mechelen with immediate effect. Dassy spent the season with Jong KV Mechelen, the club's promising team in the Second Division.
In July 2023, Dassy signed a three-year contract with SK Beveren, which had just missed out on the title in First Division B the season before.
