It’s a good time to be a Mets fan. Since becoming the first team in major league history to build a five-game division lead after only 12 games, the Mets won a franchise-record 16 games in April to open a six-game lead over the rival Braves and Phillies. In fact, Baseball Prospectus’s adjusted playoff odds report gives the Mets a 70% chance of winning the NL East and a 74% chance of making the postseason, even at this early hour.

The Amazins have their concerns, of course, like the second-base situation, the historical imperative that Paul Lo Duca will wither down the stretch, the left-handed setup corps, and the back of the rotation. Foremost among these, however, are the rotation worries.

The front of the unit, of course, is excellent: Tom Glavine is throwing seven innings a night of 2.29 ERA ball, Pedro Martinez, despite constant hand-wringing over his health, has been on the disabled list only once since 2001 and is 5-0. In all, the Mets lead the National League in ERA and the majors in strikeouts. But beyond Martinez and Glavine, the outlook is much less certain.

The fact that the Mets are relying on the middling likes of Steve Trachsel, Brian Bannister, and Victor Zambrano is a function of what they lost during the winter. Last season, Kris Benson and Jae Seo combined for 264.2 solid innings of 3.36 ERA ball. On top of that, the loss of Roberto Hernandez, a valuable force in middle relief last season, prevented the Mets from moving Aaron Heilman into the rotation for 2006.

In Benson and Seo, the Mets lost more than 50 runs of VORP (Value Over Replacement Pitcher, a Baseball Prospectus metric that measures, in runs, what a given pitcher provides over and above what a readily available “replacement” pitcher would allow in the same time), which is roughly the equivalent of what reigning AL Cy Young winner Bartolo Colon gave the Angels last season. Suffice it to say, that’s a significant loss.

As for how their replacements are faring, it’s a mixed bag. Zambrano has been customarily awful despite his dominance of the light-hitting Nationals on Monday night. Trachsel has been unspectacular (4.79 ERA in five starts), and Bannister was a pleasant surprise (2-0, 2.89 ERA in five starts) before pulling his hamstring and going on the DL. Let’s explore what’s ahead.

Zambrano can’t be that bad for much longer (whether by regression to the mean or loss of his job), but his prospects for becoming an above-average starter aren’t promising. Since coming to the Mets in July 2004 as part of the Scott Kazmir trade (a deal already recognized as one of the dumbest in Mets history), Zambrano has logged a 4.45 ERA and walked an unseemly total of 94 batters in 200.1 innings. That’s awful control, but he’s done a passable job at keeping runs off the board. Still, that long-anticipated step forward since Zambrano became one of Rick Peterson’s charges has yet to occur.

The 35-year-old Trachsel missed most of last season after undergoing major back surgery. But since becoming a Met prior to the 2001 season, he’s posted a solid 3.91 ERA. His weakness has always been the home run, but Shea Stadium cuts down on power to a significant degree. The question is whether he can hold up. Working Trachsel in the fifth spot would reduce his workload, and that’s important for the Mets. Barring injury, Trachsel should provide roughly league-average innings, which is what you want from the back end of the rotation.

The unknown quantity in all this is Bannister, who is expected to rejoin the rotation in a couple of weeks. The 2003 draftee out of USC (and the son of Floyd) showed excellent command and strong strikeout ratios at every stop in the minors. Still, some scouts have questioned his ability to adapt to the majors because of his lackluster off-speed stuff and inconsistent curve. Bannister does change speeds well, but his lack of an out pitch means he’s not the kind of hurler who can get by on a day when he doesn’t have his best stuff. That sub-3.00 ERA won’t hold up over the long term, but there’s a chance Bannister will be a credible mid-rotation guy, especially if he’s able to cultivate a better changeup on the fly.

Overall, despite the hot start, it’s a thin unit. If Zambrano doesn’t improve substantially, the Mets have the option of moving Heilman into the rotation (something that should’ve been done this spring) and deploying Zambrano as a reliever. If someone gets hurt, Heilman also becomes a starter, and the Mets are forced to dig up right-handed middle relief on the trade market (fortunately for them, that’s not a tall order).

The particular sources of concern are Zambrano’s health and effectiveness, Trachsel’s health, and Bannister’s stuff, particularly once advanced scouting reports on him start making the rounds. Heilman is as good a fallback option as one can hope for, but he’s the only fallback option. Also, as mentioned, making him a starter would leave a void in the bullpen.

The back of the Mets’ rotation isn’t a fatal flaw, but as the season stretches into the dog days of summer and the Phillies and Braves start making their runs for the postseason, it could become a troublesome weakness. But more importantly, it won’t take a “perfect storm” of misfortunes for it to completely sabotage the Mets’ playoff run. That’s something for the front office to keep in mind as the season deepens and the playoffs become something to be lost rather than gained.

Mr. Perry is a writer for Baseball Prospectus. Fore more state-of-the-art commentary, visit www.baseballprospectus.com.