Here is a filthy secret about young stars: They don't generally improve. Baseball fans have it in their minds that a player will, at 27, be a better version of the player he was at 21. On average, that's true. This chart, for example, is a bit technical, but shows that the typical hitter will, at 27, be about 10 percent more valuable per plate appearance than he was when he was six years younger.
What defines a great player, though, is that he isn't anything like an average one. And Justin Upton is a great player, or close. Two years ago, when he was 21, he hit .300/.366/.532, good for an adjusted OPS of 129. In the last 30 years, just eight other hitters have done as well by that age: Alex Rodriguez, Albert Pujols, Ken Griffey Jr., Tim Raines, Rickey Henderson, Jason Heyward, Miguel Cabrera and... Tom Brunansky. That's five players who are or one day will be in the Hall of Fame, one who's on course to join them, a player who turned 21 in August, and... Tom Brunansky. Upton's prospects are obviously high.
Take our other young stars as guides to what may be in store for the lucky owner of Upton's contract over the next five years. From ages 23 to 27, Rodriguez's adjusted OPS of 153 was actually lower than the 160 mark he posted at 20. Griffey, Raines and Henderson all hit basically the same at those ages as they did at 21, while Brunansky hit much worse. Only Pujols and Cabrera hit new levels.
None of this is of course any knock on these players. Once you're hitting like a Hall of Famer, there is no real improvement you can make, unless you're Albert Pujols and thus capable of hitting like Mickey Mantle rather than Hank Aaron. (Scoop: St. Louis has a good first baseman.) The point is just that you can't expect the kind of linear improvement from a historically talented player that you can from a merely excellent one. Baseball is hard, and going from great to greater is in many ways harder than going from good to great.
I'm a bit concerned by the sample size here, but I think the general point is useful: A player who's great at 21 or 22 just doesn't have much room for improvement. When Ted Williams hit .406, he was 22. When Al Kaline won his only batting title, he was 20. When Joe DiMaggio hit 46 home runs (his career high), he was 22. Three of Eddie Mathews' very best seasons came when he was 21, 22, and 23. When Boog Powell led the American League in slugging percentage, he was 22 and would never hit quite as well again.
Then again, it's not clear that Upton really belongs in this sort of company. His adjusted OPS (OPS+) at the age of 21 was not historically significant. The last 30 years, yes. But players don't reach the majors as early as they used to; hence, the small sample size. Aside from Upton's 2009, there have been 56 player-seasons including an OPS+ of 125 or higher for players 21 or younger.
A number of players account for more than one player-season: Kaline (2), Frank Robinson (2), Jimmie Foxx (2), Ken Griffey, Jr. (2), Mel Ott (3), Mickey Mantle (2), Rogers Hornsby (2), Sherry Magee (2), Ted Williams (2), and (of course) Ty Cobb (3).
Does Upton really belong in this company? His career OPS+ is 112. That's impressive for a player who just turned 23. But 112 for a 23-year-old outfielder ... There have been many, many players with that profile who did not become superstars.
Which leaves me, if anything, more pessimistic about Upton than Marchman is. I think that Upton will probably improve, and become a big star. But he's not a great player yet, and it's not obvious that he'll become great. If it were, the Diamondbacks probably wouldn't be shopping him at all.