Belgian defender Gregory Mertens died on Thursday, three days after collapsing while playing for Lokeren's reserve side.
Mertens suffered heart failure on the pitch and received CPR before being transported to a hospital in Genk, where he was placed in an induced coma.
Lokeren confirmed the 24-year-old died on Thursday afternoon.
The club said doctors tried to operate on the player on Tuesday, but there were more complications.
Met droefheid melden wij u het verlies van Gregory Mertens. Wij...
BRUSSELS -- Lokeren defender Gregory Mertens remained in critical condition on Wednesday, two days after he suffered heart failure during a training match.
The 24-year-old player in Belgium's top division collapsed early in Monday's game, and has been kept in a coma as doctors continue to give him emergency treatment.
Lokeren defender Gregory Mertens is in a critical condition after suffered a suspected cardiac arrest during a match.
Former Belgium under-21 international Mertens was representing Lokeren's reserve side in a game against Genk on Monday when he collapsed and received CPR on the pitch.
M'Baye Leye scored the only goal of the game as Lokeren condemned Metalist Kharkiv to a sixth consecutive defeat in Europa League Group L.
Neither had anything tangible to play for in this final outing, with Lokeren unable to sneak into the Round of 32 and their Ukrainian hosts rooted to the foot of the table.
Metalist might have salvaged some pride had they registered a first point but Leye's 15th-minute header and a resolute goalkeeping performance from Boubacar Barry ensured that would not happen.