Vaessen went down after a collision with Ajax forward Brian Brobbey, causing panicked reactions from his team mates as they frantically called for medical help.
Screens were put up around the goalkeeper to shield him from the public eye and players from both sides watched with tears in their eyes as a defibrillator was rushed on to the field.
Vaessen lay motionless on the field for minutes before he was carried away on a stretcher after regaining consciousness. Both teams went inside and after deliberations the game was called off.
“Etienne was knocked out for a while,” RKC director Frank van Mosselveld told broadcaster NOS about an hour after the incident.
“Our medical staff started reanimation straightaway but it looks like it wasn’t a problem with his heart. He was conscious again when he left the field but he did not know where he was.”
Ajax were leading the game 3-2 when play was stopped. Van Mosselveld said RKC’s players were in no condition to resume the game.
“The players close to the incident were broken,” he told ESPN Netherlands. “They broke right there on the pitch. Other players broke down when they got inside. You can imagine the impact.
“We brought everyone together, and quite fast we decided together with our captain Michiel Kramer that there was no option to resume the game.
“Although we knew at that point that Vaessen was conscious again, I was talking to the referees and our captain came to me to say that they were so emotional that they could not play anymore.”
RKC Waalwijk confirmed that Vaessen was continuing his recovery in hospital on Sunday.
“Following our update last night, we can report that our goalkeeper Etienne Vaessen had a good night and has continued his line of recovery,” RKC said in a statement.
“For now, it is important for Etienne to recover peacefully in the coming hours and days. We would therefore ask that he and his family be given the time and rest to do so.”