The actual degree / severity of the damage would be linked to the penalty, i.e. Light ( 0 to -20) / Moderate (-21 to -40) / Severe (-41 and worse) or similar (and should probably fit in with the standard HARP penalty thresholds / categories).
Thus, a -10 'generic' critical could equate to either:
a) Organ = Bruised lung
b) Bone = Fracture / bone chip
c) Muscle = Sprain
d) Tendon = Strain
whereas a -30 'generic' critical could equate to either:
a) Organ = Lung tissue damage
b) Bone = Break
c) Muscle = Incomplete Tear
d) Tendon = Stretch
whereas a -50 'generic' critical could equate to either:
a) Organ = Collapsed lung
b) Bone = Shatter
c) Muscle = Complete Tear
d) Tendon = Snapped
(I have no 'real' medical knowledge, so these might not be the best descriptions...).
Taking several 'little' injuries could (depending on the total penalty) then increase in severity...
This works best if you track by location (but obviously adds a bit of complexity)...
It is true that the 'type' of damage may not be important to YOU (as you are 'just' feeling the pain), but MAY be important for the poor sap who is trying to heal you (mis-diagnosing and treating your 'real life' injury as a 'break' rather than 'nerve' damage could introduce complications)?
You are correct to say that this may be 'too modern' and 'too knowledgable', but depends on how accurate / specific that a 'fantasy healer' could (or would have to) diagnose.
I like gritty games where injuries hurt and are a hinderance (and not immeadiately solved by magic), so this fits my personal taste...
As I mentioned, I also use the RM / MERP herbs (as I run my games in Middle Earth).
Many describe their effects as 'heals minor sprains', 'heals torn tendons', 'mends fractures (but not breaks)', etc., so this type of method helps makes the individual herbs more individual and useful.