If a player is injured after the 80th minute and has to leave the field, and all substitutes have been used up, the other team must nominate a player to leave the field to even up the numbers. Simple.
Hmm, but then instead of subbing to waste time, players will just roll around on the floor even more to waste time. And then get up. So by forcing subs to be before minute 80, the net amount of time wasted might actually increase.
I still like the "Health and Safety" rule, which is that if a player goes down injured and is treated by any medical staff, then he has to be substituted.
Perhaps an even stricter version aimed at curbing wasted time as well as diving : if a player is "injured" or otherwise lying down for more than 10 seconds, he has to be substituted. Let's call it the boxer's clause.