Six days after the draft, Jiříček signed a three-year rookie contract with Columbus, and with bonuses he could earn a total of $5.85 million. He would have an annual income of $950,000 in the first team and $82,500 on the farm. Jiříček played four duels in preparation and scored one assist.
In the past season, Jiříček played 29 games in the extra league for Plzeň and scored 11 points for five goals and six assists. But then, on December 27, in the opening match of the prematurely ended U20 World Championship against Canada, he injured the ligaments in his left knee and only returned to the game at the start of the preparation of the Czech national team before the WC in Tampere and Helsinki.
He then played five games in the championship and recorded two points for a goal and an assist. In August, at the postponed Junior WC, he collected four points for four passes in seven duels.
In three seasons in the Czech Extraliga in the colors of Pilsen, he managed 69 games and scored 21 points for eight hits and 13 passes. Participant of one U18 and two U20 world championships, he has 18 games in the national A-team and seven points for three goals and four assists.
He made his debut on November 8, 2020 at the Karjala tournament in Helsinki. There, coach Filip Pešán also fielded ten juniors against Russia, which consisted of a squad of 20 players. At the time, Jiříček was the youngest in the history of the independent national team at the age of 16 years and 345 days. Jaroslav Drobný, who made his debut in February 1937 at the age of 15 years, three months and 30 days, remained the record holder in the history of Czechoslovak hockey.