In 2009, Grzelak became playing assistant manager of the small suburban Stockholm club
IK Frej when they played in division 2 (fourth tier). He became head coach and sporting director for the club in 2012 when the club was playing in Division 1 Norra (third tier). In 2014, Frej won promotion to Superettan for the first time in club history. The team finished 14th in their debut season (2015) but retained their place after winning the relegation play-offs. In 2016, IK Frej finished in 10th place — their best-ever league finish. Grzelak left the club in the summer of 2017 to join AIK’s coaching staff. In June 2017, he joined
Allsvenskan side
AIK as an assistant manager to
Rikard Norling. The team finished second behind Malmö FF in the 2017 Allsvenskan. In the 2018 season, AIK won the league, becoming Swedish champions for the 12th time. In December 2018, he was appointed assistant manager of the
Sweden national under-21 team. In July 2020, he was appointed as AIK's new head coach, following the dismissal of Norling. At the time, the team was in 12th place in Allsvenskan. After a strong second half of the season, AIK finished in 9th place. In the 2021 season, AIK finished second, level on points with Malmö FF, who won the title on goal difference. In August 2022, Grzelak was dismissed as the head coach of AIK after a string of poor performances in August, including a 3–0 defeat away against
1. FC Slovácko in the
2022–23 UEFA Europa Conference League play-off round, as well as a 1–0 defeat against
Kalmar FF and a 2–2 draw against
IFK Värnamo in Allsvenskan. With ten games left in the 2022–23 season and the club sitting in the relegation zone, Grzelak led the team to safety. In the 2023–24 season, Fehérvár finished fourth in the league, securing qualification for the UEFA Conference League.
Újpest On 14 June 2024, he was appointed as the manager of
Újpest. He debuted with a 2–1 defeat against
Puskás Akadémia in the first round of the
2024–25 Nemzeti Bajnokság I at the
Szusza Ferenc Stadion. After a disappointing start to the spring phase of the 2025 season, the club announced on 3 May 2025 that he had left his position by mutual consent.
Cracovia On 20 April 2026, Grzelak took over as Polish
Ekstraklasa club
Cracovia's manager after agreeing to a two-year deal. He replaced
Luka Elsner, who had left them in 13th in the league table, three points above the relegation zone. == Personal life ==