With the season approaching its halfway mark, most of the divisional races have already shaken out pretty well. Every race has its element of drama, but the NL Central features suspense that the other divisions lack, because to a great extent, the eventual winner might boil down to a few questions: who’s hurt, when will they be back, and how good will they be once they arrive?

Will the returns of stars like Chicago’s Derrek Lee, Houston’s Roger Clemens, and St. Louis’s Albert Pujols alter the balance of power in the division? What about the Cards losing Mark Mulder? Are the Reds really going to make a race of it without any such major reinforcements? And can the Brewers rebound once they get Ben Sheets back, or will the return of their staff ace come too late to make a difference?

We can probably dispense with playing up the Cubs and Lee’s return. Last season’s MVP-caliber performance aside, Chicago is too broken to get back into the race from 14 games with July approaching. So we’re really talking about a four-team race, made more interesting still because none of the teams is really that strong in any one particular area.

Generally, the team-wide performances suggest that the NL Central has four closely matched ballclubs at the top of the standings.

The Cardinals have the biggest issues in terms of who’s coming and who’s going because of the quality of the players in question. Tony La Russa’s squad is depending on Albert Pujols to return to form quickly to help them compensate with two potential problems. First, there’s the continuing question about how well Jim Edmonds will hold up if he continues to get banged up. The Cards’ lineup relies on three star players to do the real damage: Pujols, Scott Rolen, and Edmonds. Losing Rolen undermined their bid to do damage in the postseason last year, and not getting Edmonds back up to full strength hurts them every bit as much as losing Pujols for an extended period of time.

That brings us to another potential problem: the Cardinals’ rotation’s fortunes now that Mark Mulder’s shoulder has given out. Admittedly, with an ERA over six, Mulder wasn’t doing the Cardinals many favors. Still, his track record is such that La Russa might have expected better, and in a rotation where both Jason Marquis and Jeff Suppan aren’t helping, and where the always-unpredictable Sidney Ponson fills the last slot, the team’s starting pitching has mostly been Chris Carpenter and little else. Losing Mulder helps in that it creates an opportunity to start the organization’s best prospect, Anthony Reyes, but that was already being considered at Ponson’s expense. By season’s end, that might lead to a better rotation, but the Cardinals definitely need to get better work from both Marquis and Suppan, and they need Mulder back at full strength.

The biggest splash among the new arrivals in the division is the relaunching of the Rocket, but Clemens doesn’t really fix a problem as much as he enhances an area that was already pretty solid. You might even worry about this making things worse: Phil Garner has probably picked the wrong youngster to pull out of the rotation, because rookie Fernando Nieve has been the team’s second-best starter in the early going.

It also depends on what you’re expecting from the great one: Baseball Prospectus’s preseason projection for Clemens was that his ERA would jump up from last year’s 1.87 to 3.04. That’s great, but not world-beating great. What if Roy Oswalt’s back problems get worse, and what if Andy Pettitte never rounds back into form? Then it won’t matter how good Clemens does – one good starter does not a rotation make. Until the Astros start getting good hitting from corner outfielders Jason Lane and Preston Wilson, they’re not even close to being a good enough offensive ballclub to outscore a weakened rotation.

The Brewers currently sit in fourth, and for a pretty obvious reason: their rotation. With Doug Davis slowly improving, Milwaukee can count on three starters in five slots, but in the other two, they’ve already tried eight starters, including a spin with recent Met castoff Jeremi Gonzalez. What they really need is to get staff ace Ben Sheets back in working order; but Sheets only recently started throwing extended side sessions, has yet to throw breaking balls, and there’s still no idea of when he’ll be able to begin a rehab assignment, let alone return to the mound in Milwaukee. Without him, a Brewers team that could otherwise run with the big dogs will be hard-pressed to stay within striking distance.

And the Reds? Outside of getting young third baseman Edwin Encarnacion (.270 AVG/.364 OBA/.481 SLG) back from the DL, they just need to keep on keeping on, which might be difficult since they’re hoping to get a full season of greatness out of Bronson Arroyo. However, even if Ken Griffey Jr. or Austin Kearns break down in the outfield, this is a deeper team than in years past now that Ryan Freel has become the team’s supersub.

New GM Wayne Krivsky has already made a significant impact by picking up Arroyo and second baseman Brandon Phillips, and there’s a lot to be said for a GM who understands that you can help yourself with some dumpster-diving on the waiver wire. Krivsky could help Cincinnati even more by shoring up the bullpen. In the meantime, the Reds can try to keep pace with the Cardinals and see if Krivsky might even outduel Cards GM Walt Jocketty at the deadline by sneaking in a veteran starting pitcher. The margins within the NL Central are slender enough that even with the latest round of reinforcements, all four contending clubs should keep an eye on helping their bids.

Ms. Kahrl is a writer for Baseball Prospectus. For more state-of-the-art analysis, visit www.baseballprospectus.com.