PITTSBURGH -- Mitch Keller had just turned 18 years old when the Pirates selected him out of his Iowa high school in the second round of the 2014 Draft. At that point, he said, he had no idea what it meant to be considered a top prospect.

“Like, I didn’t even know what that was,” Keller said last month. “Then I’m living it for five years, and still to this day, I’m still living it. It’s really crazy.”

Keller, now 23, possesses the kind of talent that creates high hopes. Those expectations followed the right-hander through the Minor Leagues, and they will continue to be a part of his identity until he takes a permanent place in the Pirates’ rotation.

The Pirates have had a pitcher atop their list of prospects for all but one year since 2011, according to the MLB Pipeline archives. (The exception was 2017, when Austin Meadows claimed the top spot over Keller.) The complete list of those pitchers: Jameson Taillon, Gerrit Cole, Tyler Glasnow and Keller.

Taillon has been held back only by injuries, and he’s now facing another significant obstacle after his second Tommy John surgery. Cole thrived for three years in Pittsburgh, seemingly plateaued, then immediately reached new heights in Houston. Glasnow mowed down Triple-A hitters then all but flamed out in the Pirates’ rotation and bullpen, only to catch fire when healthy in Tampa Bay.

What happened to those three is far beyond Keller’s control, but the Pirates simply cannot afford to miss on another top pitching prospect. With that in mind, let’s look back at Keller’s 2019 season and what comes next for Pittsburgh’s No. 1 prospect -- a title Keller still holds but should shed as he assumes a regular role with the Pirates next year.

What went right?
If you’re a fan of fielding-independent pitching numbers, you’re likely a believer in Keller. He posted a 3.19 FIP and 3.47 xFIP, both the best marks among active Pirates pitchers at the end of the season. Statcast predicts better days ahead, too, as Keller’s .256 expected average and .389 expected slugging percentage were well below the actual .348 average and .546 slugging percentage he allowed.

Those encouraging numbers were a product of Keller striking out 65 batters, walking 16 and allowing six homers in his 48 innings on the mound. If he maintains those rates while experiencing a dip in his absurdly high .478 batting average on balls in play -- the league average is typically around .300 -- he’ll be well on his way to fulfilling his potential.

It’s easy to like Keller’s easy, mid-90s fastball. His curveball is a weapon when he can convince opponents that he’s able to throw it for a strike as well as a swing-and-miss pitch. The slider he added earlier this year has diversified his arsenal. He threw 151 2/3 innings between Triple-A Indianapolis and Pittsburgh, setting him up for a full season’s worth of work next year.

And there’s no doubt he learned a lot about how his stuff plays in the Majors by making 11 starts this season.

“Just a huge, huge, huge learning experience,” Keller said. “Everything about it was a learning experience, from the good outings to the bad ones to being sent down to being brought back up and everything in between. Just a huge learning experience.”

What went wrong?
Keller’s surface-level numbers were not good: the 1-5 record, the 7.13 ERA, the 1.83 WHIP, the 72 hits in 48 innings. He only pitched into the sixth inning once, and on that night, he worked into the seventh without recording an out. Keller ended the season still needing to prove that he belongs in the Majors, especially to fans wary of previous pitching prospects’ performance here and elsewhere.

In his first few starts, Keller became too dependent upon his fastball because hitters ignored his breaking balls when he couldn’t locate them in or around the strike zone. Opponents hit .461 with a .719 slugging percentage against his four-seamer. His changeup was a non-factor most of the season. And Keller experienced four blow-up innings of at least three runs, starting with the first inning of his Major League debut in Cincinnati on May 27.

Best moment
He went into the offseason on a high note, holding the Cubs to one run while striking out seven over five innings in his last start of the season. But his best outing overall was Aug. 23 against the Reds, the same team that welcomed him to the Majors on May 27 with a six-run first inning.

Nearly three months after his big league debut, Keller permitted only one run on six hits and a walk while striking out nine at PNC Park. He established his 95 mph fastball in the strike zone, throwing 20 of 24 first-pitch strikes on the night, while mixing in legitimate swing-and-miss curveballs and sliders. Keller also set a season-high with 20 swinging strikes; only Jordan Lyles recorded more in one game for the Pirates this year.

2020 outlook
The Pirates might not openly commit to putting Keller on the Opening Day roster due to his youth and last season’s results, but he’ll certainly have a chance to establish himself in Spring Training and run with a rotation spot if he earns it.

Pittsburgh won’t have another healthy pitcher with Keller’s top-of-the-rotation upside, and that alone should give him a chance to prove himself next season. The Bucs’ rotation was such a problem last season that they should seek upgrades wherever they can find them. But Keller needs to head into Spring Training with the goal of showing the Pirates’ decision-makers that he’s entering the production-over-potential phase of his career.