[1] In 2012, Jihlava manager František Komňacký promoted him to the senior team and he went on to make his debut for the club against Slavia Prague that year.

[3][4] On 20 August 2019, Masopust scored in Slavia Prague's UEFA Champions League first leg play-off win against Romanian side CFR Cluj.

[5] Slavia would win the second leg as well to qualify for the group stage of Europe's premier knock-out competition for just the second time in their history.

[6][7] Masopust started for Slavia in the final of the Czech Cup on 20 May 2021, but was replaced by 19-year-old teammate Abdallah Sima in the 60th minute.

[8] Sima would go on to score the only goal of the game against Viktoria Plzeň as Slavia won the league and cup double.