Arajuuri steadily made his way to become a permanent player in the starting line-up of Lech Poznań during the rest of 2014. He scored his first goal in the Ekstraklasa on 14 February 2015 in the 1–1 away draw against Pogoń Szczecin. During the 2014–15 season, Arajuuri made a total of 25 appearances, of which 24 were in the starting line-up. He scored three goals. Lech won the Ekstraklasa and the Polish SuperCup, and the team made it to the UEFA Champions League qualifiers the following season. Arajuuri also appeared in the Polish Cup final, which Lech however lost to Legia Warsaw.