Belgium under-21 international Gregory Mertens is recovering in hospital in an induced coma after collapsing on the pitch during a reserve team match for Sporting Lokeren, his club said.
The match against Genk was 15 minutes when Mertens collapsed. Belgian media said he had suffered cardiac arrest and received CPR on the pitch.
"Our defender was immediately assisted by the medical team of Sporting Lokeren and KRC Genk," a statement read.
"He was transferred to the hospital in Genk.
"Sporting Lokeren sympathises with the family and hopes for a positive outcome."
Chelsea's Belgium international goalkeeper Thibaut Courtois offered his support on Twitter, along with other players.
"Keep fighting Gregory," Courtois said.
Mertens, 24, joined Lokeren last summer from Cercle Brugge for whom he made nearly 100 appearances.