It's pretty simple...
If Grasu is fully recovered from the ACL -- since he was good enough to start going in Preseason Game 4 (injured 8/30/16) -- he'll be a firstrate backup C (on rookie contract) starting the season.
If or when Sitton or Long are injured (high probability), Bears will play Grasu at C (Whitehair to G), or Kush at G, depending on how good each has looked thus far.
Especially if it's near the start of the season... if Grasu looks about as good as Kush, they'll play Grasu because Pace drafted Grasu in the 3rd Rd and he missed all of 2016. They see him more as the future starting C, than Kush as the future starting G.
However, the later in season it is, and if the Bears appear in contention, Pace is more likely to put Kush in, and keep Whitehair at C.
Consider the trending health of Sitton and Long, there's a high probability they both go down, in which case Kush has one G position and you really want to see Grasu holding the C position as soon as possible.