Luke Weaver, budding ace

The 2017 trade deadline came and went and the St. Louis Cardinals, who on July 31st stood four-and-a-half games out of a playoff spot, did nothing. At least as far as adding from outside the organization.

Instead, the Cardinals promoted within the organization. On July 27th, the Cardinals placed Luke Weaver in their starting rotation, presumably for good after they dumped Mike Leake's contract on the Seattle Mariners.

Since that date, the 24-year-old has been one of the best pitchers in baseball.

He has posted a 2.90 ERA over five starts and 31 innings since July 27th. His 2.69 xFIP over that stretch, which adjusts for his bloated HR/FB rate (16.7 percent), is the best in baseball among starters with over 30 innings pitched.

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Photo by Jim McIsaac/Getty Images

How a few adjustments, and a sustained ability to punish fly balls, have transformed Tommy Pham into the Cardinals’ best player in 2017.

He has struck out eight or more batters in four out of his five starts, and at the same time, he’s never walked more than two a game. His K-BB percentage is fourth-best in the league over that period, behind just Corey Kluber, Chris Sale, and Chris Archer.

Not only has Weaver racked up the strikeouts and limited the free pass, he is getting batters to put the ball on the ground more than at any point in his professional career. He owns a 54.1 percent ground ball rate, tied with teammate Michael Wacha for 10th in the league over that period.

Weaver is riding his changeup to dominance. According to FanGraphs, despite having thrown just thirty-one innings as a starting pitcher this year, he has the 23rd-most valuable changeup in baseball. All 22 pitchers ahead of him on the list have thrown at least 60 innings.

He has thrown his changeup 26.4 percent of the time this year, so it's hardly a surprise when he does decide to use the pitch about once every four deliveries. And yet it has been unhittable. Batters are swinging and missing against the pitch about 17 percent of the time, and when they do put it in play, they are batting it straight into the ground. About 69 percent of balls in play against Weaver's changeup have been ground balls. As a result, batters are hitting just .152 against the pitch, with only one extra-base hit, good for a slugging percentage of .174.

His third pitch, behind his four-seam and changeup, is his curveball, which he throws about eleven percent of the time. Although it’s not a put-away pitch like his changeup, it has become a solid third look for him. Last year, batters hit .400 against his curve. This year, while throwing it slightly more often, batters are hitting just .214 against the pitch. His whiff percentage when throwing his curveball has jumped from 2.1 percent last year to 7.8 percent this year.

Weaver is technically a five-pitch pitcher, with his cutter and slider filling out his repertoire. He is still working out the kinks with those two pitches, throwing them less than three percent of the time so far this year. He is already dangerous throwing just his fastball, changeup, and curveball consistently. If he can develop his cutter or slider, or both, into a reliable fourth or fifth option, he will be anchoring St. Louis' rotation in no time.

The jury is still out on Weaver. The final month of the regular season will determine the sentiment going into 2018. If he can keep doing what he is doing now, he will be a sexy breakout pick next year.

Dylan Svoboda is a writer for Beyond The Box Score and BP Milwaukee. You can follow him on Twitter at @svodylan.