I think he has rare athletic and physical abilities above and beyond most / all English centres.
I'm the same way that Billy Vunipola can be a single handed threat from standstill, Manu demands attention from the opposition defence.
England are a power based team with a large set of forwards, however, we can be easily out muscled in the backs. Manu can help in that regard as well as being a battering ram.
However, is he a 12 or 13?
Do we want a power option at 13, or at 12?
Does his passing / vision hinder a team of he plays at 12?
Does Ford, Farrell, Manu create enough to break down the opposition
Is Farrell, Manu, JJ a realistic option?
Will Manu stay fit, and if he doesn't, how much does it disrupt us to completely change the game plan for whoever plays instead?