The Dutchman's withdrawal from the starting XI at White Hart Lane on Sunday barely 30 minutes before kick-off sparked a series of conspiracy theories, but Moyes revealed on Tuesday morning that he had been advised not to play the full-back by cardiologists.
Moyes said: "He had the statutory checks with the cardiologists and the FA panel of cardiologists looked at it and advised us really late on Sunday that he should not play until further tests had taken place."
While Moyes had no hesitation in taking the action he did for the benefit of both the player and the club, he was concerned at the late notification of a potential issue.
He said: "We are just surprised that the FA panel didn't contact me or contact my doctor until 3.55pm on Sunday afternoon. I have never in my career been put in a situation like that ever, so we had that to deal with.
"We just can't understand why the message would come to us at that time, 30 minutes before the kick-off.
"It was completely outside our control, but it was in our control because we knew we had to make the correct decision for the player and for the football club. We had to do that."
Van Aanholt has now had further tests and will travel to London with the rest of the squad for Wednesday night's EFL Cup third round clash with QPR, although Moyes is yet to make a decision on whether or not he will be involved.
The Scot, who is happy that the player has recovered mentally from a worrying experience, said: "He's had some further tests and will travel to London with us, and we will make a decision on that a little bit later."