No. 9: '07—Chris Young; '17—Elvis Andrus
The Original No. 9: Young (No. 12 BA, No. 8 BP)
Though it took longer than expected for Young to go from a 16th round pick out of high school in 2001 to debuting in late '06, his 30-30 potential and ability to play centerfield placed him 20 spots above then-Arizona teammate Justin Upton on the BP list. While Young did hit 32 homers and steal 27 bases as a rookie in '07 as the Diamondbacks won the NL West, the concerns about his swing-and-miss issues weren’t unfounded; he whiffed 141 times and hit .237/.295/.467 for an 88 OPS+ and just 0.7 WAR, which set the template for his career as a regular. Only in 2010 and '11 did he top a 100 OPS+; those two years account for 10.4 of his 17.3 career WAR, though he’s stuck around as a moderately useful lefty-mashing fourth outfielder.
The New No. 9: Andrus (No. 65 BA, unranked BP)
Andrus was just 18 years old when he made BA's 2007 list, more than two years ahead of his major league debut, both of which probably help explain why he didn't make the cut for BP (he was No. 58 the next year). On July 31 of that year, Andrus was part of the blockbuster trade that sent Mark Teixeira to Atlanta; Andrus and fellow prospects Neftali Feliz, and Matt Harrison went to Texas and eventually helped the Rangers to a pair of American League pennants in 2010 and ‘11. While Andrus's total of 24.9 WAR ranks below that of a quartet of older outfielders who made BA's list ahead of him—Justin Upton, Adam Jones, Jacoby Ellsbury and Hunter Pence—his age (28) and strong 2016 season (3.7 WAR, his highest since '13) suggest he has a brighter future ahead than any of them.