During the secret war ark, they had Banner Hulk - which is effectively Bruce Banners brain in Hulks body. Something we seem to be seeing in endgame. Professor Hulk is a combination of Savage hulks strength, Banners mind, and Grey Hulks cunning. Not the same thing. On top of this, Prof Hulk is actually consider the strongest hulk at a baseline level (disregarding the madder/stronger factor, which savage Hulk can utilize to his advantage due to being... well, savage). Or was, up until World war hulk, which seems to be simply the grey/savage combined.
Its all very confusing, but there are many Hulk's running around in the comics.One cannot simply assume something for the film; based on what they understand from the comics, because its not the same thing. Hulk heals because Prof Hulk has regenerated 90% of his body mass in comics doesn't quite work, because we don't even know what Hulk this is, let alone what his capabilities are. That being said, hopefully the arm Heals, and Hulk gets some actual fight time his next go around.