I don't think there's much between Johnstone and Sa, the latter has higher highs and lower lows imo, Johnstone is more consistent, rarely chucks one in, but doesn't have a highlight reel, like Sa.
The reason even after yesterday I think he needs replacing is his distribution is woeful and it would appear that O'Neil wants to continue playing out from the back. Against Brighton and even yesterday we struggle to get out of teams pressing us high, if Dyche is going to do that then everyone will. Sa isn't the only issue there, but he's the biggest.
Johnstone's distribution is as bad, so what's the point? Mrs Sa will also be disappointed to learn that the climate in the East Mids is the same as the West.