Only sub the injured player. No coaching during the injury stoppage. Player must sit out at least 3 plays unless you call a time out.
Nah... so the lineman that goes down to allow himself to be subbed by a nickle back wanted out anyway, misses third down and maybe a punt. Big whoop. Same a for skinny wide receiver going down as the play clock runs down to get subbed by a tight end and avoid a delay penalty. Three minutes, not three plays at a minimum prefer 5, that is easy to enforce with the medical team keeping his helmet.
SO, one more time my FIXALL....
You get 5 clock minutes, with medical clearance at the little tent required for reentry, no coach rubbing dirt in it or giving them a Gatorade or salt pill. This is for offense or defense unless you take a team timeout while player is on the field or is deemed to be catastrophic as he leaves the field. You have a minimum of 70 guys available. Player safety you know.
Regardless......
1 Only replace the injured player and then the other team gets to sub 1 player to stop the team with the injury gaining ANY situational advantage by the sub. One each in that sequence is fair.
2 Second and beyond stoppages for a player is for 10 minutes. , can't be putting compromised players back out there. The rest remains the same...
EASY, CLEAN, SIMPLE, DONE DEAL All the refs have to do is keep up with number of stoppages each half. Maybe have a way to buy a player back after second injury without timeout.
I would consider an additional procedural down modification, if on offense you lose a down, if on defense roll the makers back for the offense when no team timeout is taken. If already a 4th down or 1st down move on. Maybe only after 2nd occurrence each half or some other trigger. NO yardage penalties.
Some variant of these together would address both main issues, getting player changes on the field/getting a rest, and stopping the clock.
Yeah there are ways to scam any system.