Anyone remember how raw Zeke was in his first year? Forsythe is essentially a 1st year player. A 7-footer too, which means he'll have some early growing pains.
The likely scenario is: Zeke will get roughly 25 minutes, and Forsythe gets somewhere between 10 - 15 minutes. KD will play Mad Scientist on occasion, playing both simultaneously. But Forsythe is not going to regularly take many, if any of Cventinovic's vacant minutes.
Hopefully Forsythe is Bardo 2.0 this season, pushing Zeke in practice, giving energy off the bench, and hopefully adding a little more offensive punch than we've come to expect from a Zeke-sub. And when Zeke gets in foul trouble, it should be much less of a drop off at the 5 spot than replacing Zeke with Egner or even Tree.