He did NOT have a heart attack at least according to news reports. A heat attack is when the arteries on the outside of the heart that feed the heart muscle the blood it needs is blocked and the heart fails to get the blood/oxygen it needs. A cardiac arrest is when the heart stops beating and goes into fibrillation. That is what is being reported as happening.
Here is what NewsMax is reporting:
Buffalo Bills defensive back Damar Hamlin, who collapsed on the field during Monday night's NFL game in Cincinnati, had to be resuscitated twice after suffering cardiac arrest, according to his uncle.
"His heart had went out so they had to resuscitate him twice," Dorrian Glenn told CNN. "They resuscitated him on the field before they brought him to the hospital and then they resuscitated him a second time when they got him to the hospital."
Hamlin, 24, got to his feet after he tackled Cincinnati Bengals receiver Tee Higgins in the first quarter and almost immediately collapsed. In front of a national television audience, Bills trainers immediately rushed to Hamlin's side and administered CPR and used an automated external defibrillator.
The Bills said Hamlin's heartbeat was restored on the field and he was taken by ambulance to the University of Cincinnati Medical Center, where he is in intensive care in critical condition. What caused Hamlin to go into cardiac arrest is unknown.
Glenn, who said he watched the game from Pittsburgh, Hamlin's hometown, said doctors told him Hamlin is "flipped over on his stomach" in the hospital to help take pressure off his lungs so they do not have to work as hard. Glenn said the next step is to get Hamlin, who is sedated and on a ventilator, breathing on his own.
"They sedated him just to give a better chance for him to just continue to heal better," Glenn said. "We are just taking it day-by-day. It seems like he's trending upwards in a positive way."
