On 7 April 2016, AIK head coach Andreas Alm included Isak in the starting lineup on matchday 2 of the Allsvenskan in an away fixture against Östersunds FK, alongside fellow young striker Carlos Strandberg. He secured the victory for his side, scoring the second goal in a 2–0 win, thus becoming the youngest scorer for AIK in Allsvenskan history at the age of 16 years and 199 days.