Eriksen collapsed during Denmark's opening Euro 2020 group game against Finland on Saturday and was given lengthy medical treatment before regaining consciousness.

"He was gone. And we did cardiac resuscitation. And it was cardiac arrest," said team doctor Morten Boesen, who led the work in giving Eriksen treatment on the field.

"How close were we? I don't know. We got him back after one defib. That's quite fast."

Eriksen was in stable condition at a Copenhagen hospital and had spoken to teammates via video link on Sunday, team officials said.

Boesen said on Sunday it was still unclear what caused the midfielder's collapse.

"I'm not a cardiologist, so the details about why it happened and further, I will leave to the experts," he said.

He also said the 29-year-old Eriksen may not have survived had the game not been played at a major soccer tournament with top-class medical equipment at hand.

"That was completely decisive, I think," Boesen said. "The time from when it happens to when he receives help is the critical factor, and that time was short. That was decisive."

Denmark coach Kasper Hjulmand said when he spoke to Eriksen, the Inter Milan midfielder was more concerned about his teammates' well-being than his own.

"He said, 'I don't remember much but I'm more concerned about you guys. How are you doing?'" Hjulmand said. "That's typical Christian. It was good to see him smile."

The Danish players and team staff were being given crisis management assistance as they process the incident.

Eriksen may have to stay in hospital for two more days, his agent said on Monday.

"He has been joking, he was in a good mood. He is fine," the Inter Milan player's agent Martin Schoots told the Gazzetta dello Sport newspaper after visiting him.

"We all want to know what happened, he as well. The doctors are doing lots of tests and that takes time.

"He is happy because he has seen how many people care about him. He has had messages from across the world."

Australian Associated Press