Born in Ústí nad Labem,[1] Kušej signed for the youth academy of German second division side Dynamo Dresden at the age of 15.
[3] In 2020, he was sent on loan to Czech second division club Ústí nad Labem.
[4][5] On 7 November 2024, Kušej scored FK Mladá Boleslav's only goal in a 2-1 defeat by Vitoria S.C. in a UEFA Conference League league stage game.
[6] On 7 January 2025, Kušej signed a contract with Slavia Prague until December 2028.
This biographical article relating to Czech football is a stub.