Concussions can vastly vary on a case by case basis, and with that, the recovery also varies. It may have been a pretty mean concussion that took a while to recover from. That doesn't mean much at this point. He may have a dozen more concussions, he may have none. The recovery time for his prior concussion isn't indicative of him being injury or concussion prone and at that point in the season, it was very likely they were being extremely cautious in bringing him back until they decided to just put him on IR.
When you're freshly recovering from a concussion, you are more prone to a 2nd concussion, which can be fatal (second impact syndrome). When you have no shot at the playoffs, your best move is to shut down an injured star player. If he starts having more concussions or injuries, I'd be concerned, but it's way too soon.