On paper, things are going pretty well for the Mets. At 19–12, the team is tied for first place in the National League East, and a closer look at the statistics proves it has been even better than the win-loss record shows. Despite significantly substandard performances from run producers like Carlos Delgado and David Wright, the team leads the National League in runs scored. And in spite of a pitching staff perceived as having a weak rotation and a no-name bullpen in front of Billy Wagner, the Mets also lead the circuit with a team ERA of 2.99. So what’s to worry about? Not a thing really, provided nothing goes awry, because a quick look at the backup plan shows that the Mets are like a piece of beautiful waterfront property — an enviable place to live to be sure, but if the floodwaters rise, you better have good insurance. The Mets’ current policy comes up more than a little short.

Evidence of weaknesses in the foundation has already been spotted in the rotation. Orlando Hernandez has pitched more than 200 innings in a season only once in his career and has already made his near-annual trip to the Disabled List. Beyond him, options are thin. The first shot was given to 34-year-old Chan Ho Park, a player whose promotion was seemingly based more on his 13 years of big league experience and his ready availability than any success he was having at Triple-A New Orleans, since the Korean righty had a lofty 7.29 ERA in four starts. After giving up seven runs during four innings, that experiment ended after one game. Up next was Jorge Sosa, a 30-year-old journeyman who had far more impressive numbers in Triple-A (1.13 ERA); Sosa delivered 6.2 quality innings against Arizona in his Mets debut. Still, Sosa is essentially cut from the same cloth as Park — he’s a journeyman, cast off by the Devil Rays, Braves, and Cardinals. He’s not the next big thing, nor is he some sort of diamond in the rough. The Mets have certainly made the right choice in riding the hot hand by turning to him, but with players like Sosa, that hand only stays hot for so long.

The Mets really didn’t have many other options after Sosa. The rest of the rotation in the Big Easy consists of a 2004 first-round pick, Philip Humber, who has struggled with consistency while posting an ERA near 5.00; Adam Bostick, a former Marlins prospect who with a 6.28 mark is nowhere near anything the Mets can trust, and another former Marlin, Jason Vargas, who pitched surprisingly well in the second half of 2005 for Florida, but has fallen off the face of the earth since. It would take just one more injury, or El Duque needing yet another extension to heal his ailing shoulder, for this rotation to become cause for concern. Tom Glavine has been positively Glavine-esque — an aging superstar who rarely dominates, but usually keeps your team in the game. John Maine has been shockingly good, but every bit of statistical analysis and every scouting report shows that no matter how much he has improved, he’s not going to maintain anything close to a sub-2 ERA in the long haul. And what about Oliver Perez? Who knows, but it’s hard to find a pitcher in the big leagues whose performances are more maddening, not just from game to game, but from inning to inning.

The fourth rotation slots works well enough when Hernandez is healthy, and this brings us to fifth starter Mike Pelfrey. The 2005 first-round pick has struggled mightily in five starts. Pelfrey has yet to record a victory, amassing an ERA of 6.39. He has gone six innings or more just once, and has yet to record more than three strikeouts in any one outing. How can this be for a 6-foot-7-inch right-hander who possesses the best fastball of anybody in the rotation, and a player who was generally considered the best college pitcher available when selected out of Wichita State? The answer is both simple and a matter of deep concern. Pelfrey’s heater is a thing to behold, not just for its mid-90s velocity, but also for its heavy sinking action — one that creates plenty of groundball outs. It’s hard to find such a combination of speed and movement. That said, it’s his only pitch, and though that one pitch was enough as an amateur, the lack of depth in his arsenal has caught up to him.

Coming out of Wichita State, Pelfrey had a decent-at-best curveball, and the Mets decided to scrap it in favor of a slider. Given Pelfrey’s size, arm angle, and power, it seemed a logical decision. After his performance last year in the Arizona Fall League — a final proving ground for many prospects on their way to the majors — team officials praised the progress he’d made with the pitch. However, scouts still saw a sweeping pitch that broke across the plate, but didn’t much drop, keeping the ball on a single plane and thus allowing opposing hitters to feast on it. At the same time, Pelfrey’s changeup has always been little more than a show-me pitch, and during the few times he’s shown it this year, opposing batters have treated it rudely, including when declining Rockies journeyman John Mabry deposited one well over the fence to break open an April 25 loss to Colorado.

It’s really one of the great lessons of major league baseball: Man cannot live on fastballs alone. No matter how hard you can throw a baseball, big league hitters are good enough to drive it. There are countless numbers of strongarmed young guns in the minor leagues struggling to move up because they have no other pitches with which to fool hitters and keep them on their toes.

Mike Pelfrey has all the potential in the world to help the Mets reach the postseason this year and beyond, but he’ll need to refine his secondary offerings in order to prevent opposing batters from sitting “dead red” on him. The improvements will need to come via on-the-job training as well, because the Mets’ depleted farm system leaves the team with no other choice.

Mr. Goldstein is a writer for Baseball Prospectus. For more state-of-the-art commentary, visit baseballprospectus.com.