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 wensen de familie en vrienden alle sterkte toe. pic.twitter.com/pV0lDz1UnD
- Sporting Lokeren (@KSCLokeren) April 30, 2015
The club in Belgium's top division said, "With sadness, we announce the passing of Gregory Mertens."
A statement on Lokeren's website hailed Mertens for his "quiet modesty" and said it would assist Mertens' family in any way possible.
"Now we will have to learn to live with the loss," the statement said.
Earlier this week, club doctor Kris Peeters said previous tests on Mertens' heart revealed nothing unusual.
"Gregory Mertens' heart has undergone extensive tests," Peeters told RTL. "We always do that at the club, and it's necessary to be able to play in the Europa League. This very unfortunate incident could not have been predicted.
"Gregory Mertens is an elite athlete and is only 24 years old. That makes things difficult to accept and to understand for a lot of people."
