Scott Young suffered chest pains at half-time during Glentoran's shock Irish Cup defeat by Amateur League side Newington Youth Club, club chairman Terence Brannigan has revealed.
Young stepped down as Glentoran manager on Sunday a day after the shock result.
"I know he did have some pain and that gave us all a fright," the Glentoran chairman told BBC Sport.
Director of football Roy Coyle will take temporary charge as the club await applications for the vacant job.
Club chairman Brannigan spoke of his sadness at Young's exit from the Oval and said that the decision was purely the Scotsman's.
"He's given his all for the club as a player, coach and manager and been with us for 14 years.
"I want to make it very, very clear that this was a decision Scott took in consultation with his family."
Terence spoke of the concern in the dressing-room at half-time during the game as a stressed Young suffered the chest pains.
"I would not want someone who I have a fondness for, to go through that kind of a worry and that kind of pain - both physical and mental.
"The stress he was under was obviously significant."
While saddened by Young's departure, the Glentoran chairman says that the club "could not have a safer pair of hands than Roy Coyle" to guide the team over the next number of weeks.
The managerial vacancy has been advertised on the club's website and applications will be taken until the end of this month.
"We will be going out to look for a replacement for Scott and Roy will be looking after things.
"We are awaiting to see what (applications) the advert brings."
Brannigan acknowledged that Young's resignation comes during a "very difficult period for us".
"We have gone through significant financial problems. We're managing that well now.
"This will not help our financial position in terms of no cup run but we've got a very good board of directors managing that."
The Glens chairman added that the club's planned new stadium project is still being "looked into" and he insisted that the East Belfast oufit still has a "great future" despite its current difficulties.
Caretaker manager Coyle insists that he will "walk away" from the club once a new manager is appointed.
"I'm there until they can get a new manager," Coyle told BBC Sport.
"I don't know how long that is going to take. That's up to the board and directors of Glentoran Football Club."
Coyle described Saturday's cup exit as an "embarrassment" and is demanding a response from the players.
"What I will want is that nobody outworks you for a start and that the players produce the passion and desire that Glentoran Football Club deserves."