People were saying the same kind of crap about Rielly and his "bum knee" during his Draft year. He's turned out fine.
Not the same thing. Once your knee is surgically repaired, it's fixed. His knee will never feel quite right, but it's not any more prone to injuries now, in fact it's usually said the repaired knee is stronger than it was before and less likely to be re-injured.
Concussions, on the other hand, result in permanent damage. Each incident makes the next one more likely, and the symptoms likely to be more severe. The brain damage one suffers when getting a concussion will also affect the player for the rest of his life and the damage from multiple incidents is cumulative. After the immediate symptoms fade, any permanent damage may not be noticeable, but it's there. His cognitive function may be impaired in some way for the rest of his life, it may not be noticeable but, it will exist. It may affect his hockey playing too (most players seem to enter a decline after suffering a concussion), although that's obviously a secondary concern.
I would MUCH rather tear an ACL than suffer a concussion even if the latter only kept me out of action for two weeks. I would really not want to entertain the possibility of going through the rest of my life having worse sleep, increased irritability, less ability to concentrate, decreased spatial awareness, added fatigue diminished problem solving abilities, or any of the other myriad issues that can plague a patient even years after just a single concussion.
Concussions are scary ****.