Kevin Correa is getting his second spring training start today. He is the only clearly healthy major league starter the Twins have. All the other likely members of the Twins rotation have questions about their health or whether they can pitch in the big leagues.
Worley, Diamond and Pelfrey are all coming off arm surgery. Worley seems to be completely recovered. Pelfrey says he is, but may not be. We won't really know until he is stretched out. In addition, the Twins have three other rotation candidates who are recovering from injuries Gibson, Harden and Perez. You can add Swarzak to that list, after his rib injuries. The remaining starters from last year, Hendriks, DeVries, Deduno and Walters, are all questionable. They are more stop gaps to fill holes, rather than part of any plan for the rotation.
If you look at the likely rotation candidates, Worley, Diamond and Gibson are the only pitchers, in addition to Correa, who will be under contract to the Twins next year. So if all four stay healthy and productive, the Twins will still need at least one more starting pitcher. In addition to the four guys above, Hernandez and Hermsen will likely be the other members of the rotation at AAA this year with shots as a 6th starter when needed. Unless they develop, the Twins are unlikely to want to rely on any of them to fill out the rotation next year.
Of course, there are young prospects like May and Meyer that may be ready to compete for a rotation spot next spring. But neither one is expected to start this season above AA. They aren't likely, much less certain, to be ready to take on a spot in the major league rotation first thing next spring.
And that is what makes Terry Ryan a genius at baseball roster construction "chess". He signed Correa to two years, giving the Twins one reliable starter. That positions him to let young pitchers and reclamation projects compete to fill out the rotation. Of course, nothing is guaranteed. Correia may get hurt or self-destruct. But he may be the only Twins starter the next couple years whose current track record doesn't make that somewhat likely.
Update: I left Liam Hendriks off the list of starters from last year and he is probably the most likely of that group to actually have majorl league success.