SURPRISE, Ariz. -- Bryce Miller wears a size 7 1/4 hat that on most days rests firmly atop his head, thanks to the bushy blonde locks that create an even tighter wrap to the cap’s crown. But by the end of what was likely his final Cactus League outing on Sunday afternoon, the Mariners’ top pitching prospect was forced to make a wardrobe adjustment.

Miller’s hat sailed off a whopping five times in the first inning, eerily similar to a young Félix Hernández in his first spring outings nearly two decades ago, though Miller’s issue was due to the PitchCom device stretching one side too wide.

It was a boyish moment for Seattle’s No. 2 prospect and MLB Pipeline’s No. 98 overall, who couldn’t help but cackle at third-base umpire Charlie Ramos during a routine examination for foreign substances between innings, after which Mariners coaches installed padding to prevent the cap from flying off again.

“It was annoying,” Miller said, laughing it off after the Mariners’ 2-1 loss to the Rangers at Surprise Stadium. “I was about to install a chin strap or something.”

Spring Training is about working through the kinks -- even, perhaps, learning what cap size to carry to the big leagues. Because Miller will be in Seattle sooner rather than later. 

In his biggest test yet, against a Rangers lineup featuring most of its starters -- and opposite Jacob deGrom, his idol growing up -- Miller surrendered two runs on four hits and two walks over four innings while striking out three and flashing a fastball that sat at 95 mph. He topped out with a 96 mph heater to Josh Jung, which ironically didn’t force his cap from his head. He also showed polish on both variations of his slider (the gyro and sweeper) and changeup. 

Miller’s notable blemish came via a solo homer to Jung in the fourth on a heater he left up, and his other run came after walking Corey Seager and allowing an RBI single to Nathaniel Lowe in the first. But after that, he punched out Adolis García and Jung to escape that jam. 

“I figured my stuff is good enough to throw to the big league hitters -- just go out and prove it,” Miller said.

Miller’s fastball caught attention throughout last year, when he took the biggest leap of any Mariners prospect -- both metaphorically and by affiliate promotions -- but it’s the fortification of his secondary pitches that has him on the MLB doorstep.

“He's got them all, with a changeup on top,” Mariners manager Scott Servais said. “It's really, really good. I definitely think it's a starting pitcher mix based on his delivery, his stuff, his demeanor, his whole [profile]. I think he's a starting pitcher. It doesn't mean he couldn't pitch out of the bullpen, but I see him as a starter.”

Servais’ distinction was deliberate, especially given that at camp’s outset he was more open-ended about whether Miller would be better suited for the bullpen or rotation. Was there a transformative moment where his projection changed?

“Yeah, the first time I saw him throw,” Servais said. “I mean, you get a feel right away how it works. He repeats his delivery. He gets after it, but it's not max effort. Some other guys you watch them throw and I'm like, 'That maybe profiles more in the bullpen.' But when you get around his makeup, I think it helps make that decision, too.”

Miller’s role in 2023 will likely hinge on need. The Mariners remarkably didn’t lose a starter to the injured list last year, but even they aren’t banking on that health for all of 2023, though they’ve ruled out a six-man rotation. Moreover, they also used 24 relievers last season, underscoring that they’ll eventually need bullpen reinforcements. President of baseball operations Jerry Dipoto has hinted that Miller could eventually contribute in relief.

For those reasons, the club will probably keep Miller in big league camp wire to wire, but his most likely Opening Day destination is Double-A Arkansas, where he had a 3.20 ERA and a 29.9 percent strikeout rate over 50 2/3 innings across 10 starts last season, holding hitters to a .191/.275/.258 (.533 OPS) slash line. 

“I think every outing I've gone out and competed and thrown it over the plate for the most part, and it plays,” Miller said. “So I'll continue to build off of each outing and wherever the next one is, I'll go out and throw it over the plate and see what happens.”