UPDATE 12:30 AM: After some discussion in the comments, I'm replacing Jeanmar Gomez with Felix Pie.
The task of setting a playoff roster is new to most of us, including me, and obviously, it's not impossible that the Pirates will win the NL Central outright, avoiding a one-game playoff. But as we creep closer to the end of the season, a one-game playoff against the Reds looks more and more likely.
Here's an early take on how I think a roster for the one-game playoff might work. First, the Pirates will be allowed to change their roster after the one-game playoff if they win, and then they'll get to set it again for a full playoff series. So, for the one-game playoff, there is no reason for them to use roster spots on starting pitchers who won't be pitching that day, unless they want to use those pitchers out of the bullpen.
Also, there are a number of rules that govern which players are eligible once rosters are fixed on August 31, but there's an easy way around them, in that if a player is on the DL as of August 31 and remains on the DL when the playoffs come around, a team can replace that player with any other player who was in the organization as of August 31. The Pirates have four players who were on the DL both on August 31 and now (Wandy Rodriguez, Jeff Karstens, Phil Irwin and Michael McKenry; Kyle McPherson doesn't seem to count), so they'll have a bunch of flexibility when they set their rosters. (Thanks, Phil Irwin!)
Therefore, you can expect the roster for the one-game playoff to be pretty different from that of the playoff roster they'd set for a full series. Here's an example from last season -- the Braves had eight infielders on their roster for the one-game playoff, and nine pitchers; the Cardinals had seven infielders and 10 pitchers. Both carried three catchers.
So here's the roster as it might appear in the one-game playoff, with Francisco Liriano the only starting pitcher guaranteed a spot. The Pirates have said they're going with a four-man rotation for the playoffs, but given the number of days off they would have between now and the end of the NLDS, there's still room to get creative. I'm guessing A.J. Burnett, Charlie Morton and Gerrit Cole will pitch this weekend's series against the Reds, and that Jeff Locke's season is over. But there's the possibility that the Pirates or Reds could separate themselves before then, rendering Sunday's game irrelevant. If that were to happen, it would make sense to have Locke start the irrelevant game in order to give the Pirates' starters more rest for the playoffs.
Here's the playoff schedule. Assuming the Pirates don't get into a one-game tiebreaker with the Cardinals for the division lead (that appears unlikely at this point, and involves more working parts than I'm willing to worry about right now), they'll get one day off before their one-game playoff matchup on Tuesday. I'm assuming they'll want Burnett to start Game 1 of the NLDS on Thursday if they win the one-game playoff, so I doubt he'll pitch. Cole probably won't pitch the one-game playoff, either, since he will have just pitched on Sunday. There is perhaps some possibility that Charlie Morton could pitch an inning in the one-game playoff. Then Cole, pitching on four days' rest, could pitch Game 2 of the NLDS and Morton could pitch Game 3, which doesn't happen until five days after the one-game playoff.
In other words, there are a ton of moving parts at work here, and we won't have a great idea where they'll fall until this weekend, and maybe not even then. But let's assume Morton is available in the one-game playoff, which would give the Pirates an extra ground ball specialist in case they need a double play. That may be overthinking things, but it makes sense to me, especially since Jared Hughes hasn't inspired confidence this year.
A pitcher the Pirates would only use as a mopup type, like Jeanmar Gomez or Jeff Locke, obviously has less utility in a one-game playoff than he would in a full-length playoff series. If the Pirates were to get in a huge hole in the one-game playoff, there would still be no reason to turn to someone they don't really trust, since there would be nothing left to play for if they lost anyway. There still might be value in having Gomez just in case the game goes deep into extra innings, but upon further reflection, Charlie Morton would be a much better option in that case, and nine pitchers is plenty.
I'm also tempted to include an extra lefty, like Locke or Kris Johnson, simply because so many of the Reds' best hitters are left-handed. But two lefty relievers, plus Liriano, is probably enough, and downgrading the roster just to have a third bullpen lefty for one game is probably too clever by half.
I personally think Bryan Morris should not be placed on any playoff roster, and I'd definitely prefer to go with someone like Pimentel, but the Pirates plainly don't share my opinion of Morris' pitching.
I originally had Travis Snider in place of Harrison, but the Pirates are going to have tons of pinch-hitting options with Gaby Sanchez and Jones coming off the bench (the Reds' starter will almost certainly be a righty), not to mention whatever's going on with Marte and Tabata. Harrison still isn't very good, but he'll give the Pirates some tactical options that Snider wouldn't, and that's probably worth something when you have the ability to stack your roster with position players like this. Pie joins the roster primarily as a pinch-running option.