Denmark's team doctor said Christian Eriksen "was gone" after suffering a "cardiac arrest" during Denmark's Euro 2020 opener against Finland on Saturday.
Doctor Morten Boesen told a news conference on Sunday a cardiac resuscitation was required. "We got him back after one defibrillator," he said. "That's quite fast."
Boesen said the player is fine and the tests he has undergone so far have all returned normal results.
But "we don't have an explanation why it happened," he said. "At the moment I can't answer that question."
Eriksen went to ground, free of contact from any opponent, in the 43rd minute of Saturday's match and received emergency treatment on the pitch. He was then transferred to hospital.
Denmark postponed their training session on Sunday, and all media activities had also been initially cancelled, but a news conference was called in the afternoon.
Eriksen more concerned "about the team"
Coach Kasper Hjulmand said Eriksen was "more concerned" about the team and his family than himself.
"He said he didn't remember much but he was more concerned about us, typical Christan. He is a hell of a player, but what a person as well. It was good to see a smile," Hjulmand said.
The game was suspended for almost two hours but restarted after news that Eriksen was in a stable condition. Hjulmand, however, admitted the team shouldn't have returned to the pitch to conclude the match they went on to lose 1-0.
"It was the wrong thing to make the decision in this case," he said. "Players were in a shock condition and didn't know if they had lost their best friend.
"We shouldn't have played"
"I have a sense we shouldn't have played. It was a very tough decision and a tough message," the coach said, but adding he was "really proud to be the coach of a team who responded the way they did."
Joel Pohjanpalo headed the goal that secured tournament novices Finland a shock win, confirmed when keeper Lukas Hradecky saved a late penalty.
Denmark sporting director Peter Moller said "in the end the decision was taken by those responsible.
"No one should blame the players. I didn't feel any pressure from (ruling body) UEFA, but I'm not sure it was the right decision to play the game, when you look at the players today and how they were affected," Moller said.
The Danish federation (DBU) said in a statement on Sunday that all players and staff members have "received crisis assistance and will continue to be there for each other" after Saturday's incident.
"We would like to thank everyone for the heartfelt greetings to Christian Eriksen," the DBU said.
Time to process events
Hjulmand said the players will be given the time they need to process events.
"It's very different from player to player from what they experienced first hand," Hjulmand said. "The look on Christian's face, his eyes and the expression on his face. The whole experience with him and his wife.
"Everyone needs to think of the pictures they have in their minds from yesterday. They will be given time, we will try to help the best we can.
"It's not just something you shake off. It's something that needs to be dealt with."