Earlier, officials said the midfielder was stable in hospital and had sent his greetings to his national team-mates.
Eriksen collapsed shortly before half-time in Denmark's opening Euro 2020 game against Finland on Saturday.
"He was gone. We did cardiac resuscitation, it was a cardiac arrest," said Boesen.
"How close were we to losing him? I don't know, but we got him back after one defib [defibrillation] so that's quite fast.
"The examinations that have been done so far look fine. We don't have an explanation to why it happened."
