Why was Justus Sheffield so good for the 2020 Seattle Mariners?

Justus Sheffield of the Seattle Mariners throw.s

BALTIMORE, MD – SEPTEMBER 21: Justus Sheffield of the Seattle Mariners pitches. (Photo by G Fiume/Getty Images)

What did Justus Sheffield of the Seattle Mariners change this season?

Justus Sheffield made huge statistical improvements this season. In 2019, he posted a 5.50 ERA and dropped it down to 3.58 this past season. Sheffield also decreased his walk rate and WHIP, which were red flags in 2019.

So, what changes did Sheffield make? First, let’s analyze Justus’ numbers prior to 2020.

Justus Sheffield 1.0

When Justus was the 31st ranked MLB prospect in 2018, scouts were raving about his 92-97mph 4-seam fastball and great slider from the left side.

Sheffield relied on his hard fastball in the minor leagues and it worked, but it did not last. In 2018, he made his MLB debut and made three appearances for the New York Yankees. In his three outings, he gave up three earned runs across 2.2 innings pitched.

Although it is a small sample size, Sheffield threw his 4-seam fastball 54.4% of the time. He threw his slider 35.1% of the time, and his changeup 10.5% of the time.

Justus Sheffield didn’t change much after joining the Seattle Mariners. Justus made seven starts and one relief appearance for the 2019 Seattle Mariners. Across those innings, he threw his 4-seamer 47.8% of the time, his slider 35.5% of the time, and his changeup 16.6% of the time.

When you don’t make adjustments, it can hurt and it certainly hurt the Seattle Mariners’ southpaw. He was able to strike out a handful of batters but was unable to minimize the hits and walks.

If you look more carefully at the data, Justus Sheffield’s fastball was just too hittable. When you are in the big leagues, a straight, mid-90s fastball is not a challenge if you know it is coming.

Sheffield’s fastball was around 93mph in 2019, and opposing batters hit .299 off of it. That’s not very promising especially when you are throwing the pitch roughly 50% of the time.

The other issue was his offspeed. It was not like his slider and changeup made up for his fastball’s lack of success, his offspeed pitches were not effective either. In 2019, batters hit .302 off of Sheffield’s slider and .320 off of the changeup.

When it comes to pitches, it is not always about the movement of the pitch. The Seattle Mariners coaching staff knew Sheffield had a great slider, but it wasn’t effective in 2019 because his other pitches did not compliment it.

His slider in 2019 had 40.8 inches of drop and 4.4 inches of break, but batters still slugged .491 against it. Sheffield’s fastball just wasn’t cutting it.

Justus Sheffield 2.0

So what was the big change? Sheffield scrapped his 4-seam fastball and learned to throw a 2-seam fastball.

According to Ryan Divish of the Seattle Times, Pete Woodworth and other Mariners staff recommended that Sheffield work on a 2-seam grip in early 2020. Justus got used to it quickly and decided to trust it. Apparently, Seattle Mariners catcher Tom Murphy worked really closely with Sheffield and deserves some credit for his success as well.

And look at what we saw… a huge improvement in numbers. Sheffield’s 2-seamer/sinker was not just some prototype pitch. It was pretty dang good considering how new it was.

His sinker had 24.9 inches of drop and 15.5 inches of break. To compare, Seattle Mariners ace, Marco Gonzales’ sinker had 18.1 inches of vertical drop and 11 inches of break in 2020.

Sheffield threw his sinker 91.8mph on average and threw it 47.4% of the time. Remember, he threw his 4-seamer 47.8% of the time in 2019, so he literally just swapped his fastball.

Justus still has a lot of areas to improve on. His sinker was rather hittable, but it helped his slider’s effectiveness. Opposing batters posted a .299 xBA against his new sinker, but only hit .189 against his slider.

It is clear that relying on the sinker helped. The movement on his fastball changed the look of his slider and changeup.

Sheffield’s adjustment is very reminiscent of Marco Gonzales’ changes. After joining the Seattle Mariners, Marco relied more on his sinker and cutter as opposed to his 4-seam fastball.

Justus Sheffield 3.0 (The Future)

Although Sheffield improved his walk rate in 2020, that is still an area to improve on in the coming years. I love Sheffield’s reliance on his sinker, but he needs to be more particular with it.

Sheffield left his sinker in the middle of the plate too often. He has great movement on it, but you still cannot leave it right down the middle.

Sheffield’s next step is to have 100% control of that sinker and be able to paint the bottom of the strike zone to generate more ground balls. If he can remain consistent with his sinker down in the zone, that will further enhance his slider and changeup which would break off of the plate.

Even further down the line, maybe he should learn a cutter from Marco. Being a three-pitch pitcher is good, but if he can learn a fourth pitch to keep the batter’s a little wearier, that could go a long way.

Justus also struggled against right-handed hitters. All of the runs he gave up in 2020 came against right-handed hitters. He gave up zero earned runs against left-handed batters. Having a cutter could work well against right-handed hitters, as shown by Marco.

Justus Sheffield has an exciting future ahead of him and changing to a sinkerballer is one huge step. He is still young and has a lot to learn. Expect him to be a crucial part of the Seattle Mariners rotation moving forward.