Aargh.
Leonard with his arm in a sling rather than a cast is probably a bad thing. If he had a broken bone, that wouldn't be good, but it would be a closed ended injury - X weeks out, and then you are healed and back. If this is a separated shoulder or some kind of ligament/tendon damage in the shoulder, these are the types of injuries that can affect you for the rest of your career. Shoulder injuries are very problematic for hockey players.
That being said, I'm just guessing what the nature of the injury is. We'll never know for sure, because the organization will never specify the injury beyond upper body.