I think it's one of those things that looks worse than it is - probably a graze and bruising rather than a cut... because if it had actually been a cut that big, we would have seen Chan with a bloody shirt when it happened... and we didn't.
It looked like Chan skated up from behind Yuzuru as well, so I figure Yuzuru didn't see him coming. Clearly Chan wasn't expecting Hanyu to launch, so it's just one of those things - an accident, nobody's fault. And a lucky escape really, if Chan hadn't reacted so quickly it could have been a lot worse...!