Just simple math dictates that they are going to need to use both Theis and Yabusele. It's a long season and you need around 6000 minutes of 'big man' time, even if you play "small ball" about 50% of the time.
Al only played just under 2200 minutes last year and I'm doubtful Brad will want to play him more than ~2400 at the top end. That means some ~3600 minutes need to be covered by Baynes, Theis & Yabusele. Baynes has never played over 1233 minutes in his career so asking him to deliver much more than the ~1600 that Amir played last year is probably a stretch. If he plays 1600, then that leaves around 1000 minutes each for Theis & Yabusele. Maybe a little less. Maybe a little more.
I note a comparison to the 2008 roster, which similarly had 4 true bigs (2 vets + 2 youngsters) eating most of the C/PF minutes in KG, Perk, Powe & Davis. They totaled up in similar fashion, with Powe & Big Baby combining for 1700 minutes. That team did later add PJ Brown and he, Scal and Pollard collectively combined for another 900 minutes of 'big' time, but my assumption is that difference today is probably made up with more small ball.
The point being, Theis and Yabusele will very likely each end up with somewhere between 800-1000 minutes, which is a pretty decent amount for a rookie. If we play less small ball than last year, they both could get more. Personally, I'm so sick of small ball I would like that.
Though at least this year, when we play 'small' it will more likely be with 3 wings and not with 3 guards (which is what we did a lot of last year).