Denmark midfielder Christian Eriksen was "gone" before he was resuscitated after suffering a cardiac arrest, the team's doctor, Morten Boesen, said in a news conference Sunday.
Eriksen, 29, is in a stable condition after he was taken to a hospital in Copenhagen on Saturday after he collapsed on the pitch during Denmark's Euro 2020 opener against Finland.
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"He was gone, and we did cardiac resuscitation. It was a cardiac arrest," Boesen said. "How close were we [to losing Eriksen]? I don't know.
"We got him back after one defibrillator, so that's quite fast. I'm not a cardiologist, so the details I will leave to the experts at the hospital."
The game between Denmark and Finland was halted in the 43rd minute following the incident and was resumed later Saturday, resulting in a 1-0 victory for the Fins with Eriksen being named star of the match.
Denmark boss Kasper Hjulmand said he thought it was wrong for players to be asked if they wanted to continue the match following the incident.
"I know it's very difficult," Hjulmand said. "But looking back it was the wrong thing to make the decision between the two scenarios to the players in this case.
"Players were in a shock condition. Players who didn't really know yet if they had lost their best friend. And they have to decide between these two things. And I have a sense that we shouldn't have played.
"I know it's difficult. It's just a feeling I have. Maybe we should have just gone on to the bus and gone home and let's see what the next days would have brought.
"That's just my feeling now, but I think it was a very, very tough decision that the players had to try to make a decision. I know it's difficult, but I have a sense that it was wrong for the players to be given this situation."
Eriksen had just played a short pass when he fell facedown onto the ground and was given urgent medical attention for around 10 minutes. UEFA announced shortly after that the game had been suspended.
The midfielder was eventually carried off to a loud ovation, with his teammates walking next to the stretcher.
UEFA said both teams had held an emergency meeting before deciding to continue playing, with Mathias Jensen replacing Eriksen in the lineup for the resumption of the fixture.
Hjulmand said the team spoke to Eriksen before resuming the game, adding that the Inter Milan midfielder was more focused on his teammates.
"Typically, Christian he was concerned about us and, of course, his family. He said 'I don't remember much' and 'I'm more concerned about you guys, how are you doing?' That's typical Christian. That just shows that these big players and their generosity. He's a hell of a player, but what a person he is as well."
UEFA president Aleksander Ceferin said in a statement on Saturday: "Moments like this put everything in life into perspective. I wish Christian a full and speedy recovery and pray his family has strength and faith."
Hjulmand said he will assess his players' readiness to play again before their Group B game against Belgium on Thursday. "We will try tomorrow to establish normality as much as is possible," Hjulmand added. "Players have different kinds of shocks and traumas and emotions.
"We will try to use the next couple of days as good as possible. I will try to get a feeling of the players. Maybe for some, the time is too short to be able to play football again.
"We will try to use it as a force to get even closer together and do our best in the next match. And I have a sense that we will be able to really get together and do our best. So try to reach normality in the training and do our best against Belgium."