[4] He was named the head coach of Klubi 04, the reserve team of HJK, for the 2020 season,[5] winning the Kakkonen Group B and gaining a promotion to Ykkönen.
[7] However, Gnistan were later granted the Veikkausliiga license and promoted via supplemental process, after FC Honka was declared for bankruptcy.
On 23 October 2023, Rantanen was appointed as the manager of Veikkausliiga club Ilves on a two-year deal, starting in 2024, and reuniting with former HJK sporting director Miika Takkula.
[8] In his first season as a manager in the top tier, he coached the team in the 2024–25 UEFA Conference League qualifiers, advancing to the 3rd round by an aggregate win against Austria Wien.
He led Ilves to finish as the season's runners-up, falling two points short of the champions KuPS.