But Taveras lost his rookie (and MLBPipeline.com prospect status) once he reached 45 days of pre-September service time on Aug. 2. Two other obvious candidates, Gregory Polanco (Pirates) and George Springer (Astros) graduated to the Major Leagues earlier in the season, taking them out of the running.
So it's time for some new names in this discussion, and they don't get much newer than Alex Jackson. The consensus top position prospect in the 2014 First-Year Player Draft, he was the first prep hitter taken when the Mariners grabbed him with the sixth overall pick. They signed Jackson for $4.2 million, the third-highest Draft bonus this year, and immediately moved him from catcher to outfielder.
Jackson definitely had the arm and agility to stay behind the plate, and while he needed to upgrade his receiving, the same can be said of almost any high school backstop. Seattle made the position change not because he couldn't develop into a sound catcher but because his offensive potential is so immense that it wanted to expedite his bat to the big leagues.
The Nationals made the same decision with Bryce Harper after selecting him with the No. 1 overall pick in the 2010 Draft. The Royals also shifted Wil Myers from catcher to corner outfielder, though they waited until after his first full professional season. Both Harper and Myers reached the Majors less than two years after assuming lesser defensive responsibilities, and while the 18-year-old Jackson won't advance that rapidly, he does have similar heart-of-the-order upside.
And that's why Jackson takes over for Taveras as the game's best corner-outfield prospect.
First and foremost, Jackson stands out for his power potential. He has a quick right-handed swing with natural loft, which combined with the strength in his 6-foot-2, 215-pound frame gives him pop to all fields. Jackson's swing can get long at times, but he exhibits good plate coverage and makes consistent hard contact, so he should hit for average as well.
With Jackson not having to worry nearly as much about his defense at a less challenging position, and also not getting beat up physically in the outfield like he would at catcher, he'll have a far greater chance of maximizing his offensive talents and will be available for 160 starts per season rather than 120. He could make it to Safeco Field as 21-year-old in 2017 and hit .280 with 25 homers on an annual basis once he gets established.
Rookie-ball numbers don't mean much in the long run, but Jackson got off to a good start in the Arizona League. He batted .289/.333/.500 with two homers and 10 extra-base hits in his first 20 games. The only negative was that Jackson lost a fly ball in the lights on July 21, and it hit him in the face and fractured his cheekbone when he tried to make a sliding play.
Jackson hasn't played since, but the Mariners hope to get him back on the field for the final week of the Arizona League season.
Though Jackson will need to work to become an average defender in right field, he has the tools and will require only some experience. His arm was an asset at catcher and will have no problem fitting in right field. While Jackson is not a blazer, he moves better than most backstops and should have average corner-outfield range.
In the other half of this Pipeline Perspective, Jonathan Mayo argues the merits of the Pirates' Josh Bell, who does have some advantages. Bell is a switch-hitter, has more of a track record because he signed three years earlier and already has reached Double-A and projects as a better defensive outfielder.
Ultimately, though, whether it's Jackson or Bell or someone else who proves to be the game's best corner-outfield prospect at this moment will come down to how much he produces at the plate. And it says here that Jackson will provide more offense than any of his challengers.