Speaking after Ryan Jones revealed he has been diagnosed with early onset dementia, Marler, 32, called the former Wales captain's diagnosis "awful".
The 83-cap England man recalled a "big blow" in a match that knocked him out.
"I was out cold and I remember being in the physio room next and the kit-man came in. I just broke down and I had no recollection of having kids," he said.
Speaking to TalkSport, Marler added: "It just really scared the life out of me."