One thing comes down to the age old question... do you weaken one position to strengthen another?
You do when it's the off season and there are few positions yet filled to be weakened.
And even if it's mid-season, if you have to shift a player in order to bring another one into the team you do so. Otherwise you're 'weakening one position to keep another one strong'. That's a saying I just made up.
There's no doubt Willie Tonga is the best centre at the club on either side of the field, and if it turns out Jacob Loko is the second best centre in the club, then you have to assign them to sides based on what is best for the team.
In that sense you have to look at the team as a whole, not just at individual positions. Is Loko left, Tonga right better than the other way around? If it is (and I suspect it would be for a couple of reasons) then I think that's clearly how they need to line up.
But there's another scenario that involves both Loko and Tonga on the left hand side of the team. Don't forget Jacob Loko spent time on the wing during last year's trials. Obviously he played left centre in every one of his first grade appearances, but that might have been to do with our total lack of quality at centre.
There's no guarantee Jacob Loko would be any less of a risk on the wing. But with his defence as it is, if he can't adapt to the wing quickly, I'd say he could spend a lot of the year at Wenty or even Toyota Cup. Learning to play centre.