Van der Gaag, 41, who has a history of heart problems, briefly lost consciousness during the clash at the Estadio do Restelo on Saturday, which Belenenses went on to win 1-0.
The Dutchman was taken to hospital for evaluation by a cardiology team, before being discharged to recuperate at home on Sunday.
"For now I'm going to stop working and I will solve my problem. I will do everything to come back as soon as possible," he said.
"I'm alive today, it was a very violent thing that happened on Saturday. I have had a heart problem for some time, before arriving at Belenenses, I have a pacemaker and a (internal) defibrillator.
"On Saturday, during the game I felt bad and I received two shocks and the device saved my life.
"For now I'm going to stop working because life is more important. My doctor is now my coach."
"The squad was out of the ordinary and made a great game on Saturday," he added.
"(They) showed once again that it's a team that's very much alive and wants to continue. I hope it continues in the same way."
