[4] Cassar made the team following training camp and played in his first career professional game on August 7, 2021, against the Calgary Stampeders.

[5] However, he suffered a hip injury in that game and was placed on the team's six-game injured list.

[7] He also made his post-season debut that year in the team's East Final loss to the Hamilton Tiger-Cats.

[7] He played in both of the team's post-season games, including his first Grey Cup appearance, where he recorded three special teams tackles in the 109th Grey Cup victory over the Winnipeg Blue Bombers.

[8][9] He played in all three post-season games, including the 111th Grey Cup where he had one fumble recovery in the Argonauts' 41–24 victory over the Winnipeg Blue Bombers.