Unlike Andy Sutton, I'm not an expert, so I have no qualifications to speak on the long-term effects of concussions, but the thing that worries me most about Sid's concussion is not how long he'll be out (which is bad enough, as he was having a year like nobody has had in almost 20 years), but how he'll be when he comes back. Fortunately, they say it's a "mild" concussion, but my observations of players coming back from concussions is that they often tend to be shadows of their former selves.
Marc Savard has been extremely ineffective this season, David Booth is not who he once was, Lindros went from star to Star (see what I did there?) and out of the league before his time. Keith Primeau was forced into retirement by a concussion. It took Erik Cole a long time to get back into the swing of things. The examples go on and on of players who have had concussions and never been quite the same, or at least took a while to get back there.
Granted this is anecdotal evidence, but it's enough to make me worry. Let's hope and pray that Crosby is Crosby again at least by April.