I also think lingering injuries are not the game breaker that one at first presumes. Prosthetics, possibly even magically prosthetics, regeneration, experimentation on troll organs, blessings, fiendish rituals etc can all solve the challenge in an interesting way.
Most of the lingering injuries are not too bad mechanically.
1) Lose an eye. You have disadvantage on wisdom (Perception) checks that rely on sight and on ranged attack rolls.
2) Lose a foot or leg. Your speed on foot is halved and you must use a can or crutch to move. You have disadvantage on dexterity checks made to balance
3) Internal injuries. Whenever you attempt an action in combat, you must make a DC 15 Constitution saving throw. On a failed save, you lose your action and can't use your reeactions until the start of your next turn. The injury heals if you receive magical healing or if you spend ten days doing nothing but resting.
These are probably the worst of them. Often the description is worth the the effect. Losing an exe or a foot is not necessarily worse then multiple exhaustion levels.
But I do think it's important to consider how to handle these - some of them need regeneration and some of the it just states magical healing. For eases sake I make lesser regeneration work for all of them (and reduce the effect correspondingly) and it's more easily available. It's a reasonably significant resource cost and if they don't have a Cleric or the Cleric hasn't prepped it, they need to find someone who does - however, most of them aren't so bad that you can't press on until then - which is important because if all they do is enforce downtime then they become a waste of time. The point of the injury is you should feel the injury.
Having lingering injuries on crits is a
bad idea. They're far to common and they tend to fall on your front line warriors disproportionately. Using massive damage the way I do has several safe guards. Because it's based on half your total hit points characters with higher total hit points are slightly more protected. And because you add the lower of Strength or Con front line warriors are slightly more protected. The Barbarian with 18 Str and 16 Con only has a 10% of picking up a lingering injury, while the Wizard with 14 Con and 8 Strength has a 25% chance.