Start Porcello on short rest? Move Justin Verlander up to pitch that game (which would also be on three days' rest)?
Jim Leyland preferred to keep Verlander on track to pitch Sunday, in case the AL Central race comes down to that last day on the schedule. (Plus, after throwing 129 pitches last night, the guy could really use the full rest.) And according to Jason Beck, Leyland doesn't plan on pitching Porcello again until the postseason (if the Tigers make it).
Perhaps Leyland wanted to go with a superhero.
This isn't the most obvious choice, as Figaro hasn't started a game since June, when he went on the DL with a sprained wrist. The longest outing on his rehab assignment was four innings. And since being activated by the Tigers, he hasn't pitched more than three innings.
Of course, this would be a relatively stress-free situation if the Tigers were to clinch the AL Central before Saturday. I'm just sayin'...