He's only signed up for the pilot, though. He'll have the initial fight in the prologue, but other than that, there aren't a lot of action scenes. (Bran's fall doesn't qualify, I don't think.) Even that initial fight shouldn't be terribly complicated visually. What he'll have to do primarily is establish the character relationships, of which there will be a bunch: Ned-Robert, Ned-Jon, Jon-Robb, Bran-Jon, Bran-Ned, Ned-Catelyn, Tyrion-Cersei, Tyrion-Jaime, etc., etc. For that, I think you'd want somebody with his kind of resume; which is not to say that I think he's the right choice specifically.

When the big battle scenes come up later in the season, they can get Felix Alcala or somebody like that.