Karlsson, 18, has scored four goals and registered two assist so far this season for Falkenberg.
Swedish club AIK swapped younger mascots for the oldest club members as they took the pitch prior to a game against Gefle.
A match between two Swedish sides was abandoned after a pitch invader assaulted Ostersunds goalkeeper Aly Keita on Monday.
Jordan Larsson has been held back from participating in Sweden's Olympics campaign by his father Henrik.
Zlatan Ibrahimovic has brushed aside questions about his club future but he has ruled out a return to Malmo.
Goals from Emir Kujovic and Arnor Ingvi Traustason secure the title, leaving club three points clear of nearest challengers.