[2] Following three successful seasons with the club from Deventer, Heerkens joined Willem II, signing a two-year contract on 13 June 2013.

[3] He made his debut for the Tricolores in a home match against Den Bosch on 4 August 2013, starting in a 2–2 draw.

Throughout the 2013–14 season, he made 28 league appearances as the club clinched the Eerste Divisie title and secured promotion to the Eredivisie.

[4] Heerkens grew into team captain of Willem II, replacing Pol Llonch and Jordens Peters in that position.

[11] In June 2023, he signed a third consecutive one-year contract extension, keeping him part of Willem II for his eleventh season.