Normally, Croc head should still be able to heal a wound to the brain too, but the biology of Lenny's attack was calculated with precision.
Lenny attacked the frontal lobe but pushed the thick needles all the way to the hippocampus, which plays an important role in memory and learning.
When destroyed, the brain no longer has a means of remembering how to heal, as that function has been taken away.
At the same time, destroying the frontal cortex also meant the destruction of the prefrontal cortex, which is responsible for decision-making, planning, and self-control, which can be very important in healing mental injuries.
With these two important parts severely damaged, it was not possible for the brain to heal its own injury in time.
It is now a terrible fact that about 75% of the brain is made of liquid. Of which even a reduction of two percent (2%) could cause irreversible consequences.