My guess is day one Johnson starts at the 4 but XF will get every opportunity to get that role and be starting by January. JJ playing SF and XF @ PG day one is also in play. Players with Xavier's ranking don't go to a school with plans to sit and learn for a year.
Similar ranked players like Matt Thomas, Georges Niang (B12), Lindell Wigginton, and Talen Horton-Tucker all started day 1. Top 60 kids usually need to prove they aren't ready be a starter first rather than prove they are first in this time of early transfer landscape.
those guys all started some their freshmen year, but generally not day 1.