Webb warned that referees may be reluctant to stop the game if they believe that a player is "crying wolf" - and any delay could have been fatal for Bolton midfielder Muamba. England's top official was refereeing the Tottenham v Bolton FA Cup match in March when Muamba fell to the ground after suffering a cardiac arrest.
Webb, at the FIFA medical conference in Budapest, said in an interview: "I turned and saw Fabrice Muamba lying face down on the floor with no-one else nearby - this was clearly a major concern and clearly something more than a normal injury."
He added: "The fact that he wasn't rolling around screaming in agony, the way he went down with no contact, meant immediately it was serious. And it was not only me - the players recognised it. You see William Gallas' reaction - an opposing player - immediately waving to the bench to come on.
"If the game had not been stopped within 20 or 30 seconds, that might have made a difference to his chances of recovery.
"One of our obligations as a referee is to try and observe fair play and keep the game flowing when we can.
"But, if players cry wolf too many times, then there is a possibility that maybe we will not react in the way we need to do based on what we saw there.
"If we come under criticism for stopping the games too many times for doctors or physios to enter the field of play then referees might be inclined not to stop the game.
"I'm not saying it's a particularly big problem but I have seen games stopped where players weren't as seriously injured as they would have you believe and that is an issue when you are dealing with something as serious as this."
Source: PA
Source: PA