Somebody's going to be playing less than 20 a night, because they have three centers and three power forwards (I'm including Grant) who all need court time. If it isn't Okafor, it'll be Noel or Embiid, and that guy's minutes (if any) will be eating into the minutes of the other two. If they try to carry all three through the entire season you're almost guaranteed to see some DNP-CDs along the way.Okafor isn't going to be playing < 20 min a game for he sake of Embiid. I know it's been a while since he's played, but Embiid is also a classic back to the basket post but just much worse at it.
However, if healthy and as-advertised, Embiid is also a much better defender than Okafor, which is going to be necessary to protect Simmons, and possibly Saric. The team can probably get away with a 20/20/10ish minutes split among the three (assuming very limited minutes where they play two centers), but there's no scenario where everyone's happy.