The Chicago Cubs have promoted prospect Javier Baez to the major leagues and he's expected to be in the starting lineup today against the Colorado Rockies, playing second base. It has been a down-then-up season for Baez at Triple-A Iowa, but the Cubs have concluded that he's learned all he can learn from the Pacific Coast League. Here's a look at Baez and my take on what we can expect.

Baez was drafted by the Cubs in the first round in 2011 from high school in Jacksonville, Florida. A native of Puerto Rico, he moved to Florida with his family when he was 12 years old. As a prep he drew comparisons to a young Gary Sheffield due to his explosive bat speed. He crushed the ball with Low-A Peoria in 2012 (.333/.383/.596 in 57 games) but had a few problems after moving up to High-A, hitting .188/.244/.400 in 23 games.

He returned to Daytona to open 2013 and tore up the Florida State League with a .274/.338/.535, 17-homer line in 76 games. Promoted to Double-A Tennessee, he heated up and finished with a .294/.346/.638, 20-homer bruising of the Southern League.

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The Cubs opened him with Triple-A Iowa this year. He had problems at first, hitting .172/.238/.379 with a 4/22 BB/K in 58 at-bats in April. However, he began to turn things around in May (.250/.303/.435), hit well in June (.275/.345/.471) and has been excellent in July (.300/.344/.655). He's hit 12 homers in his last 32 games and sports an overall line of .260/.323/.510 with 23 homers, 34 walks, and 130 strikeouts in 388 at-bats. He's also swiped 16 bases in 24 attempts.

Baez is listed a 6-0, 190, a right-handed hitter born December 1, 1992. Although his swing mechanics aren't textbook, Baez has one of the fastest bats I've ever seen in the minor leagues. The ball simply leaps off the wood when he makes contact, giving him plus (even plus/plus) power to all fields. His bat speed really is Sheffield-esque. However, Sheffield was much better at making contact, so the parallel does not hold in terms of their approaches.

Baez's feel for hitting is erratic: he will control the strike zone fairly well at times, but isn't consistent about that and will chase pitches outside the strike zone more often than not. Triple-A pitchers took advantage of this aggressiveness early in the season, but Baez responded to the challenge and made some adjustments according to Iowa observers, showing more adeptness at handling off-speed pitches in recent weeks.

Defensively, Baez made a lot of progress at shortstop; his arm is strong enough for the position and his error rates are improving as he irons out his footwork on routine plays. I think he could stay at shortstop at least in the short run, but the Cubs are going to use him at second base. He acquitted himself well in limited action at the keystone in Des Moines, and while he needs more experience there he should be an above-average defender eventually, perhaps an excellent one.

His pure running speed is average to slightly above but he's an aggressive runner who isn't afraid to make a steal attempt or take the extra base. Indeed, Baez's entire field presence is aggressive, highly-competitive and extremely confident, sometimes to the point of annoying the opposition. The thing is, he has been able to channel that aggression into success and making adjustments when he's needed to make them.

Although his performance at Iowa hasn't been perfect, Baez seems to respond best when challenged. He's now facing the biggest challenge of his life: major league pitchers who enjoy nothing better than carving up overconfident young hitters. Ups-and-downs should be expected, and given his past track record a cold start should not be a surprise. However, I doubt he was going to learn anything further in the PCL, so giving him two months in the majors to finish 2014 seems like a good way to get his feet wet and prepare for '15.

What kind of player will he be? Despite the bat speed, I don't think Baez will become a Sheffield-like hitter: Sheffield's career high in strikeouts was 83 and Baez will certainly whiff a lot more than that.

If you want a "dream on this" comp, imagine a player who hits like Giancarlo Stanton while playing above-average (or better) defense at second base. That's what Baez can be if he maxes out his ability.