You're talking about Connor MacDavid, I assume? The reason the Sabres aren't promoting him to their lineup is because he isn't their property. MacDavid is still only 17 years old, he's undrafted.
He's the top prospect for the upcoming 2015 NHL draft, along with American prospect Jack Eichel. With as bad as the Sabres are this year, they seem to have an inside track on MacDavid, though the Hurricanes are currently even more futile. Barring injuries, though, MacDavid certainly will be tearing up the NHL by this time next year for whatever lucky team gets to draft him.
Well, Lindros kept coming back too soon during the wild west of brain injury treatment and getting re-concussed with monster deathblows. Crosby, I believe, had a recurrence of symptoms from his original problems at the final stage of his recovery.