Mariners No. 3 prospect George Kirby exceeded all expectations in 2021, skyrocketing past several highly touted pitchers on the organizational depth chart and reaching Double-A in his first full professional season. If he keeps pitching like this, the ascendant righty might not need many more reps at the level.

Kirby was sensational Wednesday in his second start of the season, scattering two hits over five scoreless innings to pace Arkansas’ 8-0 win over Springfield. The right-hander struck out eight and didn’t walk a batter, retiring his final 12 hitters in order at Dickey-Stephens Park.

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"Right off the bat, I was really controlling my slider well in the zone and expanding it well," Kirby said. "I had a good feel for it, was throwing it hard, getting good swings and misses. When everything is working and I'm throwing strikes, I kinda feel unstoppable, honestly."

It was the latest promising development from Kirby, whose prospect stock has risen continuously since the Mariners made him their first-round pick out of Elon in the 2019 Draft. Reaching Double-A Arkansas last year in his first full season, Kirby sported a 2.53 ERA in 15 starts across two levels as well as improved stuff and polish. He now ranks as the best pitching prospect in a deep Seattle system that also features No. 4 Emerson Hancock, No. 6 Matt Brash and No. 7 Levi Stoudt.

He allowed two earned runs in 4 2/3 innings in his season debut back with the Travelers, but after Wednesday’s gem, may not be long for the level. Kirby said he's focused on decreasing his fastball usage with an eye toward refining his secondary pitches and being less predictable with his sequencing. Doing so should serve him well against more experienced hitters at the higher levels.

"I'm always trying to challenge myself," Kirby said. "Last year I was probably 55-60 percent fastball. The last outing, I was 35 percent. This one was probably around the same. I am constantly trying to challenge myself. I know I have a good heater, it’s hard, but anyone can hit a 98 mph fastball. I need to mix in all my other pitches and do it in different counts." 

The only thing holding Kirby back might be the close eye the Mariners intend to keep on his innings, after he dealt with some shoulder fatigue last summer. But the 24-year-old is one of the best strike throwers in the Minors, armed with an electric four-pitch mix to go along with plus command. He’s turned his low-90s heater into a fastball that sits in the 96-98 mph range and can touch 100, and features two distinct breaking balls as well as a solid changeup. But he's never thrown more than 67 2/3 innings in a season as a pro, reaching that total last summer.

Both Stoudt and Brash -- who earned a spot in the Mariners' Opening Day rotation -- threw more. But both will also have their workloads managed as Seattle looks to keep all three prospects healthy and reliable long-term. That's why it's not difficult to see Kirby being called on later this summer as high-ceiling depth, perhaps for a stretch run the Mariners expect to spend competing for a postseason spot.

"The only thing I'm thinking about is trying to stay healthy," he said. "That's the biggest goal. You can't achieve anything if you're not healthy. That's the biggest thing for me, to continue to work on my rehab/recovery work during the week, and put all my intent into that stuff."