Mertens, a former Belgian U21 international, collapsed on the pitch after 15 minutes of Lokeren’s reserve game against Genk on Monday after suffering cardiac arrest.
He received CPR on the pitch and was placed on a life support machine in Genk hospital, but his club confirmed that he passed away on Thursday afternoon.
He was 24.
After his collapse Lokeren released a statement stating that the reason behind Mertens’ heart condition is yet to be established, though they added that the defender passed the necessary health tests that UEFA require players to undergo during the off season – and indeed described him as one of the fittest players at the club.
Mertens joined Lokeren last summer from Cercle Brugge, and played 15 league matches this season in addition to several cup games, including one against Hull City last August.