i don't really follow rugby, but as i recall they don't have nearly the types of intricate plays involved that football does, with specific assignments and adjustments based on what happens during the play. that takes quite a bit of time to learn regardless of natural physical talent and intelligence. he could be a solid special teams player for first year since there isn't really plays involved and has much more equitable feel to rugby.
wouldn't be the first guy to make an NFL roster on special teams and develop into something more. obviously the size and speed projects well, then it just comes down to whether or not he can mentally reach the high level required of NFL players. not saying NFL players are geniuses in terms of "normal" stuff, but they do have very high football IQ, well usually. obviously as Lions fans we have seen some low football IQ guys posing as NFL players. LOL