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On Feb. 1, 2022, Mitch Keller went viral.

That day, Keller posted a cut-and-spliced video of a bullpen session at Tread Athletics, a private training facility in North Carolina. His combination of triple-digit fastballs and nasty sliders evoked bewilderment from the session’s observers. A former teammate called him the offseason’s “internet sweetheart.”

Keller created hype, and with hype comes expectations -- expectations that Keller would meet.

There’s still hype around Keller, albeit of a different tone. In 2022, Keller was trying to prove that he could pitch well for an entire season. In '23, he’s trying to prove that he can do it again.

“I think we saw Mitch grow into a man last year,” said manager Derek Shelton.

That growth required Keller to not just take his lumps, but to evolve.

Despite an excellent spring, Keller posted a 6.61 ERA in his first seven starts, and by mid-May, he was in the bullpen. Instead of folding, Keller got in the lab.

On May 18, he unfurled a sinker for the first time in his career. A few weeks later, he incorporated a “sweeper,” or sweeping slider, that he toyed with in Spring Training. Those two pitches proved to be a boon, seemingly giving Keller’s season a second wind.

From July onward, Keller had a 2.93 ERA and 3.75 FIP in 16 starts. At one point, he pitched at least six innings in six straight starts, the first time he had done so in his career. He ended the year with career bests for a full season in ERA (3.91), FIP (3.88) and innings (159), among other numbers. In his fourth Major League season -- second full season -- Keller finally put it all together over a full spring and summer.

The right-hander’s success didn’t derive solely from his sinker and sweeper. Last season, Keller displayed a fortitude that, by his own admission, he hadn’t always possessed; if he found himself in a jam, he had the confidence that he could escape.

For Keller, the priorities of this offseason have been different compared to those of last offseason.

A year ago, Keller prioritized rediscovering his velocity and cleaning up his delivery. He averaged 95.4 mph on his fastball in 2019, but that dipped to 93.8 mph. Thanks to his work with Tread Athletics, Keller’s average fastballs were back in the mid-90s in 2022.

This offseason, by contrast, Keller hasn’t needed to prioritize velocity or delivery. Instead, he is emphasizing pitch design. In addition to working on his sinker and sweeping slider, Keller is refining his gyro slider, a pitch he believes he can use against left-handed hitters.