This is a great question, and it's one that I've asked to be walked through by the Cardinals. I've heard about five, six, seven maybe different ways they could this. First, it would be putting him in the bullpen as a reliever, and as the month of April goes along have him appear in multi-inning work. (He's warming up now as I type this.) He could go from two-inning appearances to three-inning appearances. Hard to script, but possible. He could also keep his work going on the side to maintain that 30-, 40-pitch availability. It would then take a bullpen start where he gets, say, four innings to stretch him and the next time out he'd be ready for the five inning, then if he needed to get to the 80 pitch level and they didn't have a start for him, it could happen over the All-Star break in the minors and then voila up and ready to be a starter in the majors. That was the simplest progression explained to me.
I asked if the Cardinals were concerned that "spent" some of his available innings in the minors and thus wasn't maximizing his impact on the majors with rationed innings.
The answer was it wouldn't be enough considering the benefit of having him start.
If the team is prepping him to start, the answer went, then he must be pitching really well in relief, and the team will want that success in the rotation.
The Cardinals have that wide window of somewhere between 100 innings and 140 innings-ish targeted for Reyes, and the goal will be for him to be available in September and October in whatever role he's earned.