That's absolutely a legal hit if Horton still has the puck, IMO. Even legal if Horton was in the process of dishing the puck. Definitely not legal three seconds after the puck leaves his possession. Then again, he's probably not in such a prone position if he did have the puck in his possession, so part of the injury surely had something to do with the lateness of the hit.
Seriously though, big hits are a part of hockey, and should be, but absolutely not when a guy is far removed from the play. A timing hit like that is typically legal, but also very difficult to pull off for a defender. Like I said the other day, Rome should ask a big, legal hitter like Nik Kronwall how one is supposed to make an explosive check whilst a man still has the puck. Here are a couple good examples of how a similar play can be legal:
http://www.youtube.com/watch?v=SfX5BwDKjq0
http://www.youtube.com/watch?v=Lf-HK776GHA&feature=fvst
http://www.youtube.com/watch?v=wL94Y...eature=related