Kyle Seager received a text message from his old college coach, Mike Fox, after his two-homer game last week against Tampa Bay.

"You must have forgotten you're just a doubles hitter," wrote Fox, the North Carolina skipper. "What's gotten into you?"

Seager, the Mariners third baseman, replied: "Coach, I have no idea."

It was Seager at his unassuming best. Of course, he has an idea. He can measure his improvement by the amount of sweat he wipes off his forehead daily. But he'd rather shrug and act as if there's pixie dust on his bat.

Make no mistake, though: Seager is a self-made factor in this Mariners youth movement. He has risen from a third-round choice in the 2009 draft who might turn into something down the road to bona-fide untouchable building block. Right now, the 24-year-old is performing the best of the team's precocious youngsters. Seager is leading the Mariners' regular starters in slugging percentage (.495), on-base-plus-slugging percentage (.805) and RBI (17). Seager, who has a .289 batting average, also is tied for the team lead with eight doubles and four homers, three of which have come in the past week.

If you thought he was the likable little guy who just doesn't have enough pop to start at third base, you judged him too soon. Let's not make any definitive judgments based on one-fifth of the season, but if Seager keeps this up, he'll be a key part of the solution to the Mariners' much-publicized offensive woes.

He's a steady defensive player, too. Though considered a natural second baseman, Seager looks comfortable at the hot corner. Instead of pigeonholing Seager, it's best to see him the way Fox does: He's just a ballplayer, a baseball nut who will figure it out. We shouldn't underestimate the kid.

"He's one of those kids who will surprise you," said Fox, who also coached Dustin Ackley at North Carolina. "I just wouldn't sell Kyle Seager short anywhere. I know people look at his size and body type and make certain assumptions, but nothing that kid does will surprise me."

Fox was in Anaheim last July when Seager made his major-league debut. At 6 feet and 185 pounds back then, Seager looked like a young boy, with "pants that looked two sizes too big for him," Fox said.

Now, Seager is up to 195 pounds and driving the baseball regularly.

He spent the offseason adding bulk and improving his core strength. After hitting .258 in 53 games last season, Seager knew he had to get bigger and stronger to be a factor. He knows he can hit. He has been a quality hitter at every level, but the doubles-hitting machine was mostly a singles hitter a year ago. Only 16 of his 47 hits were for extra bases. Seager, who essentially jumped from Class AA to the majors, had to change that to be taken seriously as a big-leaguer.

"I needed to hit the ball harder," Seager said. "As simple as that sounds, that's what I needed to do. At this level, major-leaguers, even when they do mishit a ball, they have something behind their swings so that they can still get it into the gaps."

Aside from taking time off for his wedding and honeymoon, Seager spent the winter working on his strength, speed and agility. What's the baseball equivalent of a gym rat? That's Seager. He grew up in the game. His father, Jeff, played in college. His younger brother, Justin, is a college infielder at Charlotte. His youngest brother, Corey, is a 6-3, 200-pound shortstop receiving buzz as a possible first-round selection in the 2012 MLB draft.

Even though he's amid a hot streak, you could find Seager shagging fly balls while the pitchers were taking batting practice Monday afternoon. Then, he took some extra batting practice. Seager never stops learning, never stops trying to improve.

"That's how I have to be," Seager said. "For me to be successful, I have to outwork people."

So, as Fox told you, it should be no surprise that Seager defied the notion that he would be the odd man out in a preseason competition at third base. Mariners manager Eric Wedge generally doesn't like to play the left-handed hitter against lefty pitchers at this stage in his career, but even that will change soon. Seager is too good. He deserves the opportunity to be a true full-time player.

"I think he's a smart player," Wedge said. "He has the aptitude. And he's confident. He has hit everywhere he's been."

The Mariners are certain to have him stay in one place now and prosper from the damage he can do. Get creative imagining the possibilities. Seager is quite good at exceeding expectations.

Jerry Brewer: 206-464-2277 or jbrewer@seattletimes.com. On Twitter @JerryBrewer.