Pretorius isn't an 8, he's a 6. Although he may well play there due to lack of options if Faletau is away.
There is a dearth of genuine 8's in Wales, Faletau doesn't have much competition at all with Delve the other option not available, and the fact Powelly is probably the third best 8 says it all about the depth of 8's at the minute. It would be useful for Wales actually if Faletau didn't make the Lions tour and Heaslip, Denton and Ben Morgan go ahead and he could play for Wales in June.
FWIW, you're not going to learn much about a genuine 8 in Japan anyway. Japan have one of the physically smallest teams in world rugby, especially in the backline. A genuine 8 would have a field day, but it wouldn't be representative of their ability.
Also, there are two options: fast-track someone who could develop into a decent 8 (Wales will get away with it against Japan imo), or shun what I think is a bit of an outdated model anyway, and use someone who isn't a genuine 8 there.
The first strategy is a long-term one with no real guarantee that it will speed up the development, or the player chosen to develop will actually ever develop into a decent 8. I also think that rushing a player into developing in a problem position puts a lot of pressure on that player, and it's just better to let it happen naturally. If a natural 8 develops for Wales later, good. If not, look at ways in which Wales can get around this.
The second strategy, imo, allows Wales to vary their options in the backrow. Since Wales usually play the "archetypal 6 - archetypal 8 - archetypal 7" system, and they can only have one player on the bench, which is usually another "archetypal 7", then I think that against Japan would be a good chance to consider how they play their plan B. Having a genuine 8 on the field in games where you can't seem to get your hands on the ball can sometimes be pointless. Perhaps not always with Faletau, because he plays a lot like a flanker anyway, but having another 7 on the field can certainly be a way of trying to vary your options in the last quarter of the game. (Often, teams which are being beaten do like-for-like swaps. Tipuric-for-Warburton, imo, is a bit silly, except for when Warburton is playing badly. It simply doesn't do anything to change how the team plays.) So I'd consider, as a test for the future, an "archetypal 6 - archetypal 7 - archetypal 7" backrow: Shingler, Tipuric, Lewis.
Besides, I doubt it will be long before we see teams playing with dual opensides regularly. The breakdown is important, opensides are in fashion (in the last few years, blindside-to-openside has to be the most popular position conversion), and it's just waiting for someone to try it. I'm hoping England will consider giving it a go with Armitage and Wood. Australia with Pocock and Hooper. Wales with Warburton and Tipuric. etc.
But yeah he has been superb this season imo. Scored 2 tries, and got man of the match on his debut in the LV Cup in a 31-3 victory over Tigers. Phil Bennett sees big things for him too. Only problem is he's qualified to play both for Wales and Scotland, so Wales management need to snap him up quickly. Positive is that he is still only 24 years old. I think he has a big future ahead of him, if he is developed in the right way.
