[2] After starting his career in Finland, Koskela was snapped up by Championship side Cardiff City but he failed to impress at the club making just three appearances in all competitions, including an FA Cup game against Arsenal.

[6] In April 2005, Koskela and Njazi Kuqi were both temporarily suspended from the international duties by the Finnish FA, due to leaving the U21 team's hotel in Prague without permission from the head coach Markku Kanerva.

[9] Koskela led Klubi 04 to win the Kakkonen Southern Group, but the team eventually lost in the promotion play-offs to GrIFK.

In his first year as a head coach in the Finnish premier division, Koskela led RoPS to win a silver medal with one of the smallest budget in the league.

HJK parted ways with Koskela on 13 July 2023, after a 1–0 win over Larne FC in the UEFA Champions League qualifiers, following a relatively poor run of overall form in domestic results.

[18] Later Koskela revealed in Finnish media, that prior to his departure, the club had extensive financial problems to pay salaries for the players and the staff.