If Zach Eflin (or Thompson, or Asher, etc.) is dominant in the spring, and in the early going in Lehigh Valley (where they're probably headed), and Buchholz is "meh," we could see Buchholz moved to the 'pen (as he was in 2016 by the Sox)? There, he would be "blocking" the likes of Severino Gonzalez.
If Buchholz is sharp, he'll likely be traded. If he's not sharp, he'll eat innings, until one of the prospects takes his rotation slot away. The real uncertainty is whether Mackanin will move Buchholz to the 'pen in favor of one of the kids, if he's mediocre, and one of the prospects forces the issue.
But I have no problem having him here; I have no problem with an Eflin (or U-pick-em) having to take the starting job away, rather than seeing one or more of these kids just dropped into the rotation.