At major pro events there usually is more than one referee for a couple of reasons. One of them is that or if a referee becomes injured or incapacitated for some reason like what you mention in your question. The other reason why is if a card has 15-18 fights on it, which is not unusual for a big pro event, then two or more referees can share the work load. Moving around the ring and refereeing 15-18 fights is a lot of work physically and also it becomes very tedious keeping your senses at a heightened alert to watch and oversee things for that many fights with little or no break in between.
At major pro events that I have judged at in the past they even have multiple sets of judges for this reason as well as at least and sometimes as many as three or even four referees at a major event.
I've never seen anything like that in MMA. With that said, all the events I've seen have had multiple referees at the event. The athletic commissions in charge of assigning officials usually assigns three or more, depending on the number of fights on a card. While I don't know if they have a particular referee standing by, I wouldn't think it would be too hard to move another one into position. In fact, I've often seen a referee at ringside checking the fighters' gear before they step through the ropes.
In a mix martial fight, one fighter accidentally knocked the referee out. Will the fight continue? Is there a reserved referee to take charge if situation like this happens?
