This is probably a better discussion to have; it's the significantly more pressing issue. For me, they'll have to slot in at 13. In my mind Ford-Faz is bolted on at 10-12 and no powerful back three players have put their hands up. It'll be interesting to see how Earle and Cokonasiga develop, but they won't be getting a real look in this RWC cycle. So that leaves 13.
The 2 stand out options are obviously Te'o and Tuilagi, but both come with significant flaws. Te'o was fantastic on the Lions tour and I think has the quality to be an England starter but to back someone who will be 34 by 2019 is a pretty major gamble and so although I wouldn't rule him out, I'd try to avoid him if other options are available. Tuilagi is the most devastating centre on the planet when he's in form but form is obviously the major issue. Even if he's able to go uninjured, he's gotten far too big. He looks lethargic when he's running, as if he's trying to beat defenders with weight rather than with power - almost Bastareaud-esque. Backing such an injury prone player to redevelop the level of pace and explosiveness that made him so dangerous prior to his injuries would again be a pretty major gamble.
For those reasons, if we are to slot a powerful ball carrier into the team then it'd make sense to look elsewhere. The player that immediately stands out to me is Sam Hill. He's been seriously impressive since returning from injury; immensely physical in both attack and defence. Beyond that he's actually got a much better distribution game than he's given credit for and pretty mean footwork. I'd also expect him to be significantly faster than both Te'o and Tuilagi. The issue with him is of course that he has no experience at international level and with such strong competition for centre slots at Chiefs isn't even guaranteed to be starting for his club every week. Also the fact that he hasn't been selected for a single England squad since being an injury replacement for Slade in the 2016 Six Nations shows that EJ has no real interest in him.
So in conclusion, as it stands we have no reliable power runner to come into the side and so this was a waste of a post. Oops.