Breaking down Merrill Kelly, Diamondbacks NLDS Game 1 starter

The Diamondbacks swept the Brewers in the National League wild card round which saw them use only one of their two aces in Zac Gallen. The club’s other ace, Merrill Kelly, who was lined up to pitch in Game 3 in Milwaukee, but will instead start Game 1 of the NLDS against the Dodgers.

Kelly is coming off of a strong back of the baseball card season where he pitched to a 3.29 ERA. But when you go below the surface, the peripheral numbers are not as pretty as he has an xERA of 4.10 and xBA of .243, both of which are right at around league average. Down below we are going to break down the arsenal Kelly relies on and what makes him effectively a league average pitcher.

Pitch #1: Four-seam fastball, 2023 usage rate - 30 percent

2023 stats - .230 xBA, .421 xSLG, .329 xwOBA

Average velo - 92.2 MPH - Spin rate - 2,345 RPM - vertical movement - 17.5 inches - horizontal movement - 7.3 inches

Amongst his five most used offerings, this is the only pitch that Kelly has been unlucky with. His xSLG and xwOBA are both lower than the slugging percentage and wOBA against it, whereas on his second through fifth most used pitches the opposite is true. The four seamer has been solid for Kelly this season for two reasons. The first being it gets pop ups at an extremely high rate of 14.2 percent. Pop ups are effectively batted balls that are always going to carry miniscule expected metrics. The second being right handed batters have struggled mightily against it as they have an xwOBA of just .286 against it. As a result, the likes of Mookie Betts and J.D. Martinez could see a healthy dosage of fastballs from him.

Pitch #2: Changeup, 2023 usage rate - 23.1 percent

2023 stats - .235 xBA, .329 xSLG, .277 xwOBA

Average velo - 88.7 MPH - Spin rate - 2,138 RPM - vertical movement - 33.8 inches - horizontal movement - 15.7 inches

The changeup is Kelly’s second most used pitch against lefties as he throws it 30.9 percent of the time to them. Lefties have “been poor” against it this season as they have a slugging percentage of .234 and wOBA of .213 against it. However, they have an xSLG of .383 and xwOBA of .304 against it, meaning Kelly is due for some significant regression on the offering. Where lefties have actually struggled with the changeup though as they have a 45.1 percent chase rate against it. This is going to create an interesting dynamic for Kelly when facing Max Muncy who has a chase rate of 35.9 percent this season against changeups.

Pitch #3: Cutter, 2023 usage rate - 18.1 percent

2023 stats - .228 xBA, .359 xSLG, .306 xwOBA

Average velo - 90.8 MPH - Spin rate - 2,350 RPM - vertical movement - 20.6 inches - horizontal movement - 1.3 inches

By no means is Kelly’s cutter a dominant pitch, but where it excels at is limiting hard contact. It has a hard hit rate (batted balls 95+ MPH) of just 26.9 percent, the lowest mark amongst his five most used pitches. Additionally, like his four seamer, he does a great job of producing pop ups with it as it carries a pop up percentage of 13.4 percent. Like his changeup, lefties have not been great against it with an xwOBA of just .318.

Pitch #4: Sinker, 2023 usage rate - 12.2 percent

2023 stats - .288 xBA, .439 xSLG, .343 xwOBA

Average velo - 92.4 MPH - Spin rate - 2,204 RPM - vertical movement - 15.6 inches - horizontal movement - 15.5 inches

This is Kelly’s worst pitch in his arsenal and frankly I am confused as to why he throws it as much as he does. It does not excel at any one thing and is likely why the usage rate on it in September was down to 10.4 percent. It does not produce ground balls at the rate you would expect it to as the 53.4 percent ground ball rate is well behind his curveball and changeup. Additionally, the whiff rate of 16.2 percent against it is the lowest amongst all of his pitches by 8.2 percent.

Pitch #5: Curveball, 2023 usage rate - 11.1 percent

2023 stats - .268 xBA, .416 xSLG, .351 xwOBA

Average velo - 82.1 MPH - Spin rate - 2,569 RPM - vertical movement - 51.0 inches - horizontal movement - 8.9 inches

Kelly’s curveball is not a traditional curve in the numbers it garners against it. It is not a swing and miss pitch as the whiff rate against is just 25.6 percent. Additionally, it does not produce much soft contact as the average exit velocity against it is 91.7 MPH and the hard hit rate is 51.3 percent. Like his sinker though, the usage rate on it dipped considerably in September dropping all of the way down to 5.8 percent.