I don't think that a rule stating that, if you went down and play was stopped as a result, you have to stay out for, say three plays, would cause major disruption...
Should this new rule apply to both defense and offense? I'd hate to be the team that has a QB dinged up during a 2 minute drive and now he's out for 3 plays. But I also don't think it would be fair to make this change only for defenses. It would suck to lose a defensive playmaker, who isn't faking, for 3 plays while offensive playmakers only miss 1. It would be just another rule favoring offenses.
Since I like old school defensive battles, while annoying, I don't really have a huge problem with the fake injuries. The game is tilted to far to the offensive side of things as it is, don't need to tilt it even more.
Instead of punishing defenses doing whatever they can to just catch a breath, how about alter the rules to slow down offenses. Actually, it's already in the rules. The officials are not suppose to make any extra effort to speed up the placement of the ball but, they all do it for HUNH offenses. If the officials would just take the same amount of time as they do for other offenses it would slow things down some. I mean, you even have officials running around and stumbling to get in place. An offense should not be aloud to snap the ball before all officials are set but it happens all the time, which is why they miss some things like false starts and lineman not set.