True "ace" pitchers in baseball are the most coveted (and rarest) of commodities. World series champions almost always have a dominant starter on the staff, sometimes even two of them. When the Arizona Diamondbacks won the 2001 World Series, the playoff MVPs were Curt Schilling and Randy Johnson. Who needs offense with a dominant mound one-two punch like that?

When the Cardinals won the 2004 NL pennant and advanced to the World Series, they had a bad bit of luck in the postseason, as ace Chris Carpenter was lost. St. Louis was without their 'top guy' in the World Series, while the AL champion Red Sox had a pair of aces in Schilling and Pedro Martinez. The result was a 4-game sweep. Last October, the Red Sox sported another ace in Josh Beckett, riding his golden right arm to another title. Without him, the Indians probably would have won their first World Series since 1948.

"Workhorse" aces have such a huge impact on a team. They are called stoppers for a reason halting any losing streak every 5 days. They also rest the bullpen, regularly going 7-8 innings. And in the playoffs, there's arguably nothing more valuable than a stopper, especially in a Game 7.

While we have yet to reach the All Star Break, health issues to stoppers are already making some of this season's pennant races particularly interesting to watch. In Chicago, almost everything has been going well for the first place Cubs, at least when they play at Wrigley. Fans are already talking about the team's first World Series title since 1908. However, those talks were put on hold this week, when ace Carlos Zambrano went on the shelf needing an MRI.

Without Zambrano (8-3) the Cubs just got swept by the Tampa Bay Rays. Left-hander Sean Marshall comes up from Class AAA Iowa to take Zambrano's turn. You can already see what a domino effect that can have on a staff. Suddenly the No. 2 starter is forced to become a No. 1, and so on. And instead of Zambrano the Cubs have to use a kid up from AAA. The good news is the Cubs still have the best record in the majors. The worrisome news is that the Cubs returned home smarting from a 2-4 trip.

Arizona has a similar concern. They are worried about ace Brandon Webb, who is 2-3 since winning his first nine starts. Webb lasted just 3 1/3 innings as the As pounded him 15-1. He allowed a season-high seven earned runs and nine hits. He struck out just three and walked five. Those arent the normal numbers of a guy who won the 2006 NL Cy Young Award.

The NL West-leading Diamondbacks were outscored by a combined 35-7 during a three-game skid. For all their woes, the Diamondbacks remain 4 games ahead of Los Angeles in the feeble NL West. Arizona is 7-14 since May 26 but have added a game to its lead in that span. They may be able to survive in this bad division, but will have vastly diminished October hopes if Webb is not 100%.

When the Cardinals advanced to the 2006 World Series, they got there by upsetting the Mets in Game 7 of the NLCS. The Mets starter for Game 7 was Oliver Perez and his 3-13 record. It didnt help manager Willie Randolphs resume to handle the rotation so a kid 3-13 starts the biggest game of the year. It was no surprise Randolph was recently fired with moves like that, plus the Mets historic collapse last September and underachieving 2008 start. He certainly could have used a true stopper the last three years.

A potentially crushing blow to the red-hot Yankees was ace Chien-Ming Wang going out for 6 weeks with an ankle injury. They already lack pitching depth and Wang is invaluable. Having an ace has been a huge part of the success of the Rays (Scott Kazmir), while the Angels have several outstanding starters that kept them afloat while the offense struggled.

The Indians have struggled despite having a pair of aces on C.C. Sabathia and Fausto Carmona. Of course, Carmona's injury hasn't helped matters. He'll be back soon (not soon enough for fans of the Tribe!) and it wouldnt be a shock to see them make a second half run with those two. The other side of the coin is that Sabathia is in his 'walk year' and could be dealt likely to a contending team that needs an ace. That could also help tip the pennant balance. Remember back in 2003 the Marlins had a terrible first three months, then roared back to win the World Series. How did they do it? A rotation of Josh Beckett, Dontrelle Willis and Brad Penny. Hows that for a trio of aces!