Who will step up as the Phillies’ No. 3 starter down the stretch?

CINCINNATI — While one No. 3, as in Bryce Harper, tested his healing thumb by taking batting practice Monday in Philadelphia, it was worth pondering a question about another: Who will emerge as the Phillies’ No. 3 starter down the stretch?

Zack Wheeler and Aaron Nola are Nos. 1 and 2, respectively, and in interim manager Rob Thomson’s perfect world, they would start Games 1 and 2 of a best-of-three wild-card series. That scenario would give the Phillies their best chance to move on. It may even make them dangerous in an ultra-short series.

Assuming, of course, that they don’t melt down in September again like ice cream on a 100-degree day.

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But what if the Phillies have to win any of their last three regular-season games in Houston to sew up a wild-card berth? If the rotation stays on turn — no injuries or unforeseen doubleheaders — Wheeler and Nola are scheduled to face the Astros in that final series. And they will pitch as long as champagne corks haven’t been popped by then.

The playoffs will begin Oct. 7, two days after the season ends. So, it’s possible (maybe even probable?) that Wheeler or Nola, or even both, wouldn’t be able to start Game 1 of a wild-card series on regular rest.

In that case, who would get the ball? Who should get it?

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The candidates — Noah Syndergaard, Kyle Gibson, and left-hander Ranger Suárez — will pitch this week against the stripped-down Cincinnati Reds. It’s too soon to consider it an audition. Too much can still happen. Maybe the Phillies will fade, as they did in each of the last four years, and this is all moot. Maybe, with only seven teams vying for six National League playoff spots, they will wrap up a wild card in time to align their rotation. Even so, they would need to decide on a No. 3 starter to follow Wheeler and Nola.

Let’s analyze the options, in the order that they will face the Reds:

In time, maybe Syndergaard’s pre-elbow surgery fastball velocity will come back. It’s possible. He’s still two weeks shy of turning 30, so eventually he may trend closer to 97 than 94.

But the Phillies are taking a shorter-term view. After picking up Syndergaard from the Angels at the trade deadline, they have him through the end of season. So, they’re focused on optimizing his post-surgery stuff, particularly his sinker and slider.

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Syndergaard is throwing the slider as hard as he did before surgery. He unleashed it 37 times last week against the Marlins, his highest total since Aug. 12, 2018. Going into Monday night’s start against the Reds, he allowed 17 hits and two walks in 11 innings and had a 4.91 ERA in two starts for the Phillies.

“I think it’s just getting the comfort and confidence in where my slider used to be,” Syndergaard said last week. “Once I get the feel for it, just got to keep on running after that. I look forward to continuing to work with [pitching coach] Caleb [Cotham].”

Quick, name the active pitcher who has made the most regular-season starts without starting a playoff game.

That would be Gibson.

Tuesday night will mark the 34-year-old right-hander’s 253rd career start, 17th among active pitchers. But his lone postseason appearance was a mop-up inning for the Twins in Game 1 of the 2019 American League Division Series against the Yankees.

Gibson could get his chance this year. He has a 2.89 ERA in six starts since July 2, when he allowed four consecutive home runs against the Cardinals at Citizens Bank Park. He also has given up three earned runs or fewer in 16 of 22 starts overall. He has pitched well twice against the Braves and gotten mixed results (one good start, one rough start) against the Cardinals and Padres, all possible first-round opponents.

Suárez went on the injured list and missed two starts in early July because of lower back spasms.

He may also have gotten a second wind.

Suárez had a 4.33 ERA in 15 starts through the end of June. Over his last five starts since returning to the rotation, he has allowed four earned runs in 28⅓ innings for a 1.27 ERA. He’s throwing strikes (25-to-6 strikeouts-to-walks ratio) and keeping the ball on the ground with his sinker.

In short, Suárez looks like he did last year when he became the first pitcher since Bob Gibson in 1968 to post a sub-1.50 ERA in at least 100 innings while making a minimum of 10 starts.

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Suárez has worked 107⅓ innings, topping his total from last season (106). It’s his highest total since 2018, when he logged 139⅓ innings between double A, triple A, and the majors.

“I’m feeling pretty good,” Suárez said recently through a team translator. “My goal is always to attack the hitters early and have quick innings.”

If Suárez holds up down the stretch, it won’t be difficult to decide who follows Wheeler and Nola — or, if necessary, steps in for them.