I played high school football and had one serious concussion. Never played football again, but I had side effects for years after.
Brain injuries are to be taken seriously.
Imagine your skull is filled with a few dozen eggs. Try injuring your neck without scrambling the eggs. Possible, but not all that easy to do.
No one would thank Muzzin for coming back and putting his health at risk. If he does have a serious brain injury, then he should hang up the skates, contribute off the ice as best he is able for the duration of his contract, collect his pay and enjoy his life, family and the fruits of his labour. He's earned that.
If his contract is insured, he should consent to a Datsyuk/Pronger-type trade that relieves the Leafs of the consequences of LTIR.
If it isn't insured, then the Leafs could open up much needed cap space with LTIR anyway, expand their roster and move on.
It might be time.