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Walker is an immense presence on the mound

Mariners' No. 1 prospect throws power fastball and has improved secondary pitches @BerniePleskoff

The past few seasons, I've enjoyed watching Seattle Mariners pitching prospects Taijuan Walker, Danny Hultzen and James Paxton as they work on the sidelines at the beginning of Spring Training. With the passage of time, I believe the right-handed Walker has the brightest future as well as the biggest upside of the three.

Looking even larger than his 6-foot-4, 210-pound frame, Walker is an immense presence on the mound. Lean and lanky, the 21-year-old may have reached his total physical maturation. Walker can stand to gain some additional upper body strength, but I think what we see is what we'll get.

Walker was a dual-sport star at Yucaipa High School in Southern California. In addition to pitching, he played shortstop alongside D-backs third-base prospect Matt Davidson. Walker was also a star basketball player.

With no college in his immediate plans, Walker signed a professional baseball contract. The Mariners selected him with their first pick (No. 43 overall) in the 2010 First-Year Player Draft.

Walker is ranked No. 1 by on the Mariners' Top 20 Prospects list. As a rookie, he began 2010 in the Arizona League, where I got my first real look at his dominating fastball. At that time, I didn't see much development of Walker's secondary pitches. That has changed markedly.

After pitching at every Mariners classification in parts of four seasons, Walker is currently pitching at Triple-A Tacoma. He has a 3.61 ERA in his 11 starts, covering 57 1/3 innings. Walker began the season starting 14 games at Double-A Jackson in the Southern League. He was dominant, throwing to a 2.46 ERA and yielding only 58 hits in 84 innings.

Walker's repertoire has evolved over time. While he still throws roughly 65 percent fastballs at a range of 94-96 mph, he has a quality 89 mph changeup to buckle the knees of unsuspecting hitters. And if that isn't enough, Walker can throw his curve at 74 mph and mix in a cutter off his fastball. He has truly evolved to a quality rotation starter with a full arsenal of swing-and-miss pitches. Walker is commanding the secondary pitches much better at this stage of his development. However, his current average of four walks per game signals more work is ahead.

In addition to his limited arsenal in the past, Walker gave me pause with his slight tendency to throw across his body. I've watched lots of pitchers that have that type of delivery find success, including former D-backs ace Brandon Webb.

At times, that type of delivery causes the ball to drift, resulting in lack of command and control -- the two most important requirements for sustained pitching success. Over time, however, I think Walker has made some adjustments to improve his extension and more consistently repeat his delivery properly.

Walker throws downhill with his long arms and legs, and -- surprisingly -- left-handed hitters have a tad more difficulty hitting against him than righties. That's a bit unusual, but much of that has to do with his arm angle and the fact that Walker is not afraid to use the entire plate and pitch inside.

With 371 2/3 innings pitched so far in his career, Walker is approaching the stage of his development where he can dispatch better quality hitters with his velocity and improved repertoire.

Walker is set to make his big league debut on Friday against the Astros, and he will bring a big arm and outstanding upside to the mound.

Bernie Pleskoff has served as a professional scout for the Houston Astros and Seattle Mariners. Follow @BerniePleskoff; on Twitter.

Seattle Mariners, Taijuan Walker