It's the same choice every night, but there were strong arguments on both sides.
Flaherty has a six-game hitting streak. Casilla has lots of success against Masterson.
Showalter chose Casilla, whose tie-breaking three-run homer in the seventh inning was the decisive blow in a 6-3 victory.
So, why Casilla?
"A lot of things," Showalter said. "Alexi's a talented guy and I'm trying to not let him sit too long. I know he's going to play tomorrow and I'd like to give him the benefit of two days in a row. And he's had a little success off Masterson. There aren't many guys that have. That was it.
"I think he sat a little longer than he should have, too, so I wanted to give him a couple days. Ryan's been swinging the bat well. Let him sit on that for a day or so and feel good about himself. So, a lot of reasons."
Chris Davis' two-run shot tied the game. He's the guy who's supposed to hit home run. But Casilla?
What ran through Showalter's mind as Casilla was hitting his first home run since Aug. 8, 2012 against Masterson?
"I feel real good for him," Showalter said. "I'm happy for him. I see all the work he puts in. And he was frustrated with the finger injury, because we were getting ready to run him out there a little bit. I think about the people who got on base before him. I think about Chris' blow. Actually, just to push the first one across, because we knew runs were going to be at a premium.
"I can keep going about the things I think about, but mostly I'm just happy for him. I know how much contributing means to all these guys."
Chris Tillman made a huge contribution by going seven innings and recording his 10th quality start after issuing four walks in the first three innings.
"Tilly worked his way through it," Showalter said. "He was, I wouldn't say 'laboring.' Just having a tough time finding his step and his rhythm, release point. And he made the adjustment. If you had said he was going to pitch seven innings and win that ballgame early on, especially the stuff Masterson was carrying tonight... He's got a chance to be special."
Is he an upper-echelon starter at 9-2?
"He's got a lot more bridges to cross," Showalter replied. "I think it's hopefully very young in his career and he's trying to establish himself as somebody that his teammates and the organization can count on. Chris has an upper-echelon makeup and approach. I'll say that. He's got some deception in his delivery, and along with that comes times when he gets out of whack a little bit. I think he's gotten a better feel for what he's got to do to get back in sync.
"He just doesn't give in. He had an extra day of rest today. Tilly's a good teammate. As you get to know him, there's a lot beating in there. Very competitive guy. He holds himself to a high standard. He does. If he pitches real well and we get beat, he's not happy. And if he pitches poorly and we win, he's happy, but he's disappointed in himself. He's a real sincere guy and I'm glad he's on our side."
Tillman's maturity was on display again as he retired eight in a row after Jason Kipnis' two-run homer in the fifth inning.
"You can see his rhythm pick up," Showalter said. "It was kind of tough when we were making three up, three down real quick and he went out there and got into some deep pitch counts, it seemed like every inning. And once he got in step, you saw him really get after it.
"The maturity is hiding from me after the seventh inning. You've always got to go look for him after the fifth inning. He gives you that look. He won't tell you he's ready to come out of a game, but for the right reasons. He's sincere."
Down on the farm, Brian Roberts went 2-for-4 with a double and two runs scored in his first injury-rehab game at Triple-A Norfolk.
Buck Britton extended his hitting streak to 21 games with a go-ahead single in the eighth inning.