Sparky decides between pitchers of the same role based on OAV. Lowest OAV comes in first, even if he's not the "best" pitcher by other standards. Without a designated closer, I'd assume Setup A would be at the top of the hierarchy to enter the game in a save situation. So the pitcher who will enter the game is the Setup A with the lowest OAV, assuming he's not below auto-rest levels on fatigue and his inning available is < or = to the inning at the time.
For example:
Setup A - '90 Dennis Eckersley - .160 OAV, 0.61 WHIP, Inning Available = 7, 100% fatigue (so auto-rest is not a factor)
Setup A - '11 Aroldis Chapman - .147 OAV, 1.30 WHIP, Inning Available = 8, 100% fatigue
7th inning or earlier: Eckersley comes into the game first since Chapman is not available until the 8th
8th inning or later: Chapman enters before Eckersley every time both are available since his OAV is lower