Right now Zeller is this team's 10th player on the rotational chart. Stevens has decided to go with athleticism on the court meaning sets with Turner at SF and Green at PF as well as Rondo, Bradley, Smart combos with Bass and Olynyk, or Sully and Olynyk, or Bass and Sully, or Green and Sully or Green and Olynyk. He likes the defensive pressure and ball movement these combos can give the team.
Zeller, being the least athletic and least skilled of the top 10 players in the rotation will often be the odd man out, though I do expect to see him play more against larger frontlines. But let's face it, he's not some world beater of a talent that is screaming out load that he deserves big minutes. He's a bench role player and will have nights he sees 20 MPG and he will see nights where he sees 5 MPG. He is what he is. No more no less.