Jonathan Richter, a 24-year-old midfielder for FC Nordsjaelland was struck by lightning non-league match against FC Hvidovre on Monday.
Doctors have since placed the player in an induced coma.
Nordsjaelland spokesman Christian Wolny said: “He is on a respirator and has been cooled down to increase his chances.
“When the doctors think it is the right time, they will increase his temperature to normal body temperature and see if they can wake him up.”
Police spokesman Brian Munch told the Ekstra Bladet that Richter may have been hit in the chest or head by the strike.
“He’s in critical condition and his relatives are with him,” Mr Munch added.