It can be difficult to distinguish between starters and relievers in the minor leagues but I am going to give it a try. A lot of this is still up in the air and will probably be dependent on spring performances but as of right now the following is what I see happening.
AAA: Brad Lincoln, Rudy Owens, Jeff Locke, Kyle McPherson, Shairon Martis
As of now I am thinking Lincoln will start off in AAA and unless another starter is signed he will be the 6th starter meaning he will be the first one called up when a need arises. Owens will repeat AAA and will hopefully have a better year positioning himself for a possible call up. Locke will start the season in AAA and is probably the second option should an additional starter be needed. McPherson will be the highest regarded pitching prospect in AAA and with a strong showing could be up in the majors by the middle of the year. Rounding out the rotation will be minor league signee Martis. Martis is still young enough where he can’t be considered just minor league filler but he is probably on his way there. If an 8th starter is needed in April or May (like Karstens, last season) it will probably be Martis.
AA: Aaron Pribanic, Mike Loree, Phillip Irwin, Nate Baker, Michael Colla
The Pirates have a bit of a gap in pitching talent and it will be present in AA this season. No one who appears likely to start the season in the rotation profiles as a top rate prospect. Loree was signed out of independent ball last year and had a good debut in AA but he profiles as a reliever. Michael Colla is in a simlar mold to a pitcher with a similar name, Michael Crotta. He profiles as a swing man who may be able to make the occasional spot starter but is better off in the bullpen. In the other 3 starters the Pirates have actual prospects. Pribanic appears ready to move up to AAA but space is limited there and keeping him in AA is not a bad idea anyway. Irwin was promoted to AA last season and put up solid but not dominating numbers so he will likely repeat the level. The only AA new comer will be Nate Baker. Baker has put up decent numbers during his minor league career, never wowing anyone but still putting up solid work. At 24 he will be pushing the upper limit for legit AA prospects so he will need a good showing.
A+: Gerrit Cole, Jameson Taillon, Zach Von Rosenberg, Colton Cain, Zack Dodson
The prize of the system will be the A+ rotation. It will obviously be headlined by Cole and Taillon. Those two are known to most Pirate fans as the future of the team. Cole will likely be on the faster path and may only remain in A+ for the first month or so but Taillon will likely be at this level for half a season at least. The rest of the rotation will be filled with high upside pitchers such as Von Rosenberg, Cain and Dodson. Those three have a long way to go in their development but they all have the talent to become legitimate major league starters. This could be a defining year for them. There will also be a lot of other arms vying for innings so expect to see many players making starts early on.
A: Nick Kingham, Trent Stevenson, Ryan Hafner, Josh Poytress, Michael Jefferson
There will be no set rotation in A ball. The Pirates use a piggyback system in which two pitchers combine to make a start. One may pitch 3 innings and the next 2 but it makes little difference who is actually starting. I am guessing the 5 players I have listed will receive the most innings early on but that is little more than a guess at this point. Kingham is the best pitching prospect at this level and Stevenson and Hafner have enough upside that they will probably be given a lot of innings but Poytress and Jefferson will likely share time with other pitchers. For the most part this will the first full year in the system for these pitchers so I will skip getting into too much conversation about them.