When the Arizona Diamondbacks dropped the last game of their weekend series with the Colorado Rockies yesterday, it was their first loss in nine games. One can get only so excited over returns this early in the season — the St. Louis Cardinals, a wretched team, are a half-game ahead of Arizona for the best record in the National League, and the Detroit Tigers, a terrific one, have the worst record in baseball — but the last week or so had the feel of a coming-out party.

Coming into the season, Arizona had to be counted the favorite in the National League West. They won the division last year, albeit despite scoring fewer runs than they allowed, and improved over the winter by trading strong but superfluous minor league talent to the Oakland Athletics for 27-year-old starter Dan Haren, giving them a second ace to back former Cy Young winner Brandon Webb. The team’s youth — six of eight regulars last year were 26 or younger, and none was older than 31 — was another good reason to think they’d improve.

The one real knock on the team was the lineup’s lack of a true, franchise-grade hitter. While Arizona’s lineup is deep and balanced, nearly all contenders have at least one top hitter. A player such as David Wright or Vladimir Guerrero can compensate for any number of failings; in the absence of one, a team can find it nearly impossible to make up for the odd injury or off-season. This is why the most encouraging development of Arizona’s young season hasn’t been the winning streak, but the development of 20-year-old right fielder Justin Upton, who’s already hit five home runs, and looks to be turning into the no. 3 hitter his team needs, right before our eyes.

If this seems an impossible bit of praise to heap on a player who after all is just enjoying a nice first two weeks, it’s worth remembering Upton’s pedigree. Younger brother of Tampa Bay Rays star B.J. Upton and the first overall pick in the 2005 amateur draft, Justin has been lauded since high school as a prodigy, one of those rare freak talents to whom normal rules of development simply don’t apply. His 2006 season was an enormous disappointment, as he did nothing at all in A-ball and earned the beginnings of a dodgy reputation. But in retrospect, it appears that he may, like Florida’s Hanley Ramirez, simply have been bored by playing at a level so far beneath his talents. Last year he shredded the High-A and Double-A classes, and at 19 found himself the starting right fielder in a pennant race.

Just watching Upton play at all, it’s easy to see how all this could happen. At 20, he has the frame of a running back in his prime, and one doesn’t need to be an experienced amateur scout to see what’s meant when people say he has the gifts you can’t teach. Like Wright, he has a preternaturally quick and balanced swing, and he seems physically incapable of hitting the ball any way other than squarely. If he wasn’t hitting a lick, you’d still want to see him playing in the majors, just because a player with such talent has to be challenged at the highest level to develop to his full potential; as is, he went 4-for-7 with two key home runs this weekend in two games in which his team thrashed their top divisional rivals 18–5.

When this sort of thing happens, it’s an enormously important thing, not just for the player but the sport. Since integration, just 53 players, a majority of them stars and many of them Hall of Famers, have hit as many as five home runs in a season at 20 years old. If Upton manages to hit a home run every 10 games the rest of the season — and he’s a solid bet to do a fair bit better than that — he’ll become just the 12th player in the last 61 years to hit 20 home runs in a season at such a young age, joining the likes of Willie Mays, Frank Robinson, Al Kaline, Ken Griffey Jr., and Alex Rodriguez.

It’s very rare that a player of this grade comes along this young and displays a full command of his talents for any length of time at all, and when it happens, no expectations are at all relevant. The most recent players to have done so are Miguel Cabrera and Jose Reyes, and both have, at exceptionally young ages, already done things that would have surprised even their most ardent supporters. Arizona’s winning streak may be over, but the arrival of what can already be deemed the franchise player is a devastating development for the rest of the National League West, and perhaps even to the claims of the Mets and Chicago to being the best teams in the league.

tmarchman@nysun.com