The development of Mitch Keller is absolutely critical to the future of the Pittsburgh Pirates. Given the struggles we saw from the Pirates' rotation in 2019, we could sure use a pitcher like Mitch to step in and be the ace of the staff. He showed flashes of the ability that made him the Pirates' number one prospect this season, but also struggled to show he belonged at times. 

Keller's 2019 numbers are tough to look at. He posted a 7.13 ERA, and a 1.833 WHIP. For those of you who are not familiar with WHIP (walks and hits per inning pitched), it means that he gave up an average of almost two base runners (1.833) an inning. Not good.

It wasn't all bad for Mitch, though. Like I said, he showed flashes. His 12.2 Ks/9 was impressive. It was his best strike out rate at any level and reaching that mark during his 48 innings in the big leagues is meaningful. When you couple that with the fact that he has yet to establish a go-to punch-out pitch, it means that number could go higher.

Mitch Keller was far too reliant on his fastball in 2019 - he threw it 60% of the time despite batters hitting .451 off of it. He also didn't miss many bats with the fastball - earning only a 9.47% whiff rate. This is why it's odd that he stuck with the heater, even when there were two strikes on the batter. Against lefties, he threw the fastball 56% of the time with two strikes and 48% against righties. Both of his breaking balls had better whiff rates than his fastball and, his slider was elite at 26.75%. For reference, Gerrit Cole throws an excellent slider, and he only gets a whiff on 22% of swings. I'm no pitching coach, but that feels like low-hanging fruit to me. If Keller utilizes his excellent slider more, especially with two strikes, it will allow him to be less predictable and improve upon an already excellent strike out rate.

One thing that Mitch has not struggled with, that many young pitchers do, is control, least not as it translates to walks. His 3 BB/9 rate is just fine, but it doesn't tell the whole story. Keller's problem with control may not be in keeping the ball in the zone, but rather controlling the ball within the zone. Teams teed off on him when he was even or ahead in counts and much of that was due to his fastball being in the heart of the zone. When in the lower levels, he could rely on the heat to overpower opponents, but it doesn't work the same at the big-league level. Locating the fastball is key, and is something Keller must improve upon.

The good news is that Keller was significantly better in September than he was the rest of the year. He continued his heavy usage of the fastball, but it drew weaker contact than in previous months — more ground and fly balls, fewer line drives. The key there was that he had better success keeping the ball down in the zone.

Mitch's improved September was almost derailed when a line drive struck him in the wrist during the second inning of a 9/3 game against the Miami Marlins. Keller left the game, but didn't miss time.

Keller is one reason the pitching coach is a crucial hire. His success is largely on the shoulders of that man, whoever he is. His stuff is too good to continue to be the guy he was in 2019 and he needs someone to come along, coach him up and teach him how to better sequence pitches at the major league level. In my opinion, they are already good enough for him to be a top-of-the-rotation starter. He just needs to properly utilize them in addition to consistently locating his fastball.

Mitch Keller will get a chance next season to improve. He should start the year in the rotation and could be counted on to anchor a rather underwhelming staff by mid-season. I would look for his slider and curveball to become bigger parts of his repertoire in 2020 as he looks to keep hitters off balance.

Follow Jared on Twitter: @a_piratelife