The one potential flaw for each contender

Achilles was the greatest of the Greek warriors in the Trojan War. As a child, his mother dipped him in the River Styx. The water made him invulnerable except for his heel, by which his mother held him. Eventually, that's how Achilles was defeated, a strike to his heel.

That pointless piece of history brings us to the playoff races. Every contender has an Achilles' heel this year. Most contenders have more than one flaw, more than one way to be defeated. There is no River Styx, no magic water, no immortality in baseball this season, especially in the National League.

Here are the contenders -- by our definition, teams that are less than four games out of a playoff spot -- and the possible Achilles' heel of each.

Mariano Rivera Rivera

American League
Yankees: Mariano Rivera. He pitched for the first time on Friday, tossing a scoreless ninth inning in the Yankees' 4-1 victory over the Devil Rays, since being sidelined on Aug. 31 with a muscle strain near his right elbow. Rivera is perhaps the greatest closer of all time, and the best closer in the history of postseason play.

The Yankees are really, really good right now, but they're obviously not the same team without Rivera at 100 percent. If he's not, the Yankees are vulnerable. If he is the Mo of old, they're the team to beat in October.

Tigers: Magglio Ordonez. It's not him, he's just a symbol of their offense as a whole. The Tigers don't have a thumper who scares or intimidates anyone in the middle of the order.

Since they were 40 games over .500 on Aug. 7, the Tigers have gone 16-26. In those 42 games, they've averaged just over four runs per game, which isn't good enough, not in October.

Twins: Boof Bonser. Again, it's not him. He is just a symbol of a team that's headed for the playoffs with three rookies in the rotation and another in the bullpen. All of them, especially Bonser, have greatly contributed.

The Twins have been the best story of the game over the last three months, a gritty, determined team that might even win the division. But in October, starting a rookie pitcher, and maybe two, against the loaded Yankees isn't an ideal situation, especially if you have to open a series in New York.

Esteban Loaiza Loaiza

A's: Esteban Loaiza. He had a fabulous August. He was the American League's Pitcher of the Month. But he needs to have a strong finish, and a terrific October, for the A's to win a playoff series. There is little in his history to suggest that Loaiza will be really good in October.

The A's win with their pitching and defense, so they'll have to outpitch everyone from here on out. And they can't necessarily count on Rich Harden, who pitched three innings on Thursday, his first appearance with the A's since June 4.

Pedro Martinez Martinez

National League
Mets: Pedro Martinez. He's made two starts since coming off the disabled list, and he's consistently been throwing only in the 85-86 mph range, which isn't good. There is still time for his calf to heal and for him to get his timing back. If he's Pedro at his recent finest in October, the Mets are going to win the NL pennant. If he's not, the Mets will have concern.

Given the expectations and the way they won their division, anything less than the pennant will be a disappointment for the Mets. A lot falls on Martinez, and there was no evidence from either of his two most recent starts that he's ready to be a good pitcher this October.

Phillies: An ace. Who is the ace of this staff? Who is the one guy the Phillies send to the mound on the final day of the regular season to win the game that takes them to the playoffs? Who is the guy who can win a game on the road to start a series in the playoffs?

Jon Lieber, Brett Myers, Jamie Moyer, Randy Wolf and rookie Cole Hamels have done a nice job keeping the Phillies in contention for the wild card, but with each day the need for an ace becomes greater. Do they have one?

Braden Looper Looper

Cardinals: Braden Looper/Adam Wainwright. Closer Jason Isringhausen is out for the season. Even though he had a wildly erratic year (10 homers allowed, 38 walks), filling his spot will not be easy. Looper has closed before; his stuff is sharper this year than last, and he's had more success against left-handed hitters.

Manager Tony La Russa will mix and match mostly with Looper and Wainwright, who likely will be a starting pitcher next season. Closer questions aren't good for a team that isn't itself offensively and doesn't have a clear-cut No. 2 starter.

Dodgers: Bullpen. It's a miracle that the pen has been as good as it's been, given the injuries to Eric Gagne and Yhency Brazoban. The bullpen is loaded with good arms and gaudy numbers, but it's also loaded with young guys -- and guys who haven't pitched in October (including closer Takashi Saito).

Padres: Petco Park. Most teams use the home-field edge down the stretch and in the playoffs. The Padres don't. Through Friday, they are 41-38 at home, 40-34 on the road. Why? The ball doesn't carry, especially in the gaps, at Petco Park. The Padres hit a lot better on the road than at home: .278 to .245.

In a way, that works to their advantage since they likely will be playing more road games than home games the rest of the season (and in October). But home-field advantage should count for something, and it doesn't seem to for the Padres.

Tim Kurkjian is a senior writer for ESPN The Magazine.