Denmark team doctor Morten Boesen says Christian Eriksen was "gone" after a cardiac arrest but medical test results have been normal so far.
The 29-year-old collapsed during his side's Euro 2020 opening match against Finland, and was given emergency CPR on the pitch with the game temporarily suspended just before half-time.
Eriksen is currently being treated at Rigshospitalet, one of Denmark's top hospitals, which is less than a mile from Parken Stadium in Copenhagen.
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"The exams that have been done so far look fine," Boesen told a news conference.
When asked what caused the incident, Boesen said: "We don't have an explanation why it happened. I can't answer that question.
"I didn't see it live, I saw it on screen when it happened. You saw the same as me. No explanation so far."
Boesen confirmed Eriksen suffered a cardiac arrest, saying: "He was gone.
"We did cardiac resuscitation, it was a cardiac arrest.
"How close were we to losing him? I don't know, but we got him back after one defib, so that's quite fast."
Head coach Kasper Hjulmand said Eriksen had told him he did not remember much from Saturday's collapse and that he was eager to get back on to the pitch.
Hjulmand quoted the midfielder as saying: "I think you are feeling worse than I am. I feel as if I'm about to go training now, boys."
"Christian is in good spirits and it's a huge relief for the players after all this uncertainty," Hjulmand said. "There is no doubt that we have been on the ropes."
Doctor and coach: Game should not have been played
Team doctor Boesen also revealed that Denmark's players and staff were visited by a psychologist on Saturday night, and in hindsight feels the game should not have resumed.
"I don't think the right decision was to play the game," Boesen said.
"We had help from a psychological point of view at the hotel last night. Everyone expressed their feelings and how they saw the situation, and everyone was pleased we did this and talked it through.
"We really appreciated the professional help we have had from the outside."
Hjulmand agreed that the match should not have been completed after the incident, and said the squad would try to use what happened as motivation for their next game against Belgium on Thursday.
"No, we should not have played," he said. "We will try tomorrow to establish normality as much as possible. Players have different reactions to shocks and trauma but we will try to get back to normal as much as possible.
"I get the feeling from the players that maybe the time is too short to try to play football again, but maybe we can use it as a force to get together and try to go out and do our best in the next match."
The Danish players have been in contact with Eriksen via video calls, confirmed Peter Moller, director of Danish football association DBU.
"This morning we have spoken to Christian Eriksen, who has sent his greetings to his teammates," a Danish FA statement read on Twitter.
"His condition is stable, and he continues to be hospitalised for further examination."
PFA: Danish team allowed Eriksen dignity he deserved
The PFA (Professional Footballers' Association) has sent its good wishes to Eriksen and his family following Saturday's events.
A statement read: "First and foremost, our thoughts are with Christian Eriksen, his partner and their family. We are so encouraged by the news of Christian's stable condition and send him our best wishes for a full recovery.
"Our gratitude goes to the referee, Anthony Taylor, and the medical team for their prompt action in enacting emergency protocol under extreme pressure, which ultimately saved Christian's life.
"Our thoughts have also been with the players of both teams, especially Denmark. Captain, Simon Kjaer and Kasper Schmeichel played pivotal roles by rushing to Eriksen's aid, assisting medical staff and coordinating the players, who did all they could despite their own distress.
"In their actions, the Danish team allowed their teammate the dignity he deserved, and their demonstration of such calm leadership commands our ultimate respect.
"The trauma the players will have experienced has been at the forefront of our minds since Saturday. Any Danish or Finnish player who is a current or former PFA member is entitled to access our wellbeing services, and we will be in touch with the players this week to offer our full support.
"The term 'football family' is used loosely at times, but on Saturday, united in our hope for Christian, that's what we all were."