The three tiers of starting pitchers at this year's trade deadline

Starting pitchers Cole Hamels (signed), Anibal Sanchez, Wandy Rodriguez, J.A. Happ, Jeremy Guthrie and Jonathan Sanchez (all traded) have come off the market in the past eight days. In other words, no impact starting pitcher has yet changed the shape of the pennant races -- though who knew Doug Fister (8-1, 1.79) would be so valuable after joining Detroit from Seattle last year?

Let's face it: Nothing is more valuable this time of year than a starting pitcher. With playoff spots often decided by one or two games -- especially this year with logjams for the second Wild Card in each league -- it makes sense to invest in literal game-changers. Starting pitchers might only get 10-12 more starts, but they can influence those games more than any other player.

Do trades for starting pitchers this time of year work? You bet. Over the past five years, contenders have traded for 19 starting pitchers in July. Those pitchers proceeded to rack up a combined record of 94-45 -- a .671 winning percentage. Nine of them wound up pitching in the postseason, including Edwin Jackson, who went 6-3 in 16 starts, postseason included, after being traded last July to the eventual world champion St. Louis Cardinals.

Other sudden impact pitchers included Cliff Lee (2010 Rangers, 2009 Phillies), Roy Oswalt (2010 Phillies), Ted Lilly (2010 Dodgers) CC Sabathia (2008 Brewers) and Joe Blanton (2008 Phillies).

Teams still doing last-minute shopping for a starting pitcher this year include the Rangers, Angels, Dodgers, Orioles, Blue Jays and Braves. With four shopping days left, here are the best available options, ranked according to how much impact they can provide:

1. Race-changers: Zack Greinke, Josh Johnson, Ryan Dempster, Cliff Lee

Greinke seems almost certain to be traded before he makes his start Sunday for the Brewers. Milwaukee likes pitchers Martin Perez of the Rangers and Julio Teheran of the Braves -- and why not? They're major league ready and figure to be rotation fixtures for years. But Greinke, though he is only 28, a former Cy Young Award winner and 25-9 over the past two years, is no sure thing to pay such a high price. Two executives from large market teams both said they wouldn't touch Greinke because of concerns that his makeup is better suited to a small market. His road ERA this year (4.09) is far worse than it is at home (2.56).

Johnson, also 28, has been durable this year, but his velocity and bite on his slider have been inconsistent and his strikeout rate has been declining. And keep this in mind: His worst and least durable month of his career is September (4.29 ERA with just 16 starts in seven seasons).

Still, he comes with a contract for next season ($13.75 million) and has ace stuff when he's right. He also has a higher price tag than Greinke, but yes, he can be had in the right deal. Said one scout, "The owner [Jeffrey Loria] there is so fed up he's ready to get rid of just about any high-priced player and start over."

Dempster, a pedestrian 117-121 lifetime, jumps into this category because he is the ultimate hot hand. He is pitching to a 2.25 ERA while allowing few walks and home runs. It's still more likely he makes his next start (scheduled for Monday) for the Dodgers, not the Cubs.

As for Lee, the Phillies signed Hamels to his extension with the thought of keeping Roy Halladay, Lee and Hamels together -- their version of Maddux, Glavine and Smoltz. Executives expect him to stay put, though don't be surprised to hear the Rangers make a run at him.

2. Rotation mortar: Matt Garza, James Shields, Paul Maholm

Cubs president Theo Epstein and GM Jed Hoyer are looking to turn any short-term asset into long-term ones. Garza has pitched well in both leagues and in the postseason, bringing a level of security to a contender. Likewise, Shields is a valuable innings eater, but the Rays are not expected to move him unless it's a clear win for them in terms of what's coming back.

Maholm is essentially the same lefthanded pitcher as Rodriguez -- only younger and hotter. Like Dempster, Maholm is a rising stock. Over his past six games, he is 5-0 with a 0.94 ERA -- looking a lot like the potential Fister of 2012.

3. Discount rack: Francisco Liriano, Kevin Correia, Randy Wolf, Jeff Francis

How does a guy who allows fewer hits than innings pitch to a 5.31 ERA? The problem with Liriano is he looks great when he controls his slider and awful when he doesn't. "He can give you seven [innings] or 2 1/3 -- that's the problem with counting on him," said a scout.

Said another evaluator, "There were a ton of teams there to watch him the other day -- and by the second inning they were all out of there."

Correia is expendable because of the Pirates' acquisition of Rodriguez, and could be worth a flier for another NL team. His low strikeout rate is a warning to AL teams. Wolf, after an awful first half, has put together back-to-back quality starts.

Francis may be the best bargain out there, with Toronto heavily interested in bringing the lefthander back to his home country. He's been so good lately that the Rockies let him blow through their 75-pitch limit in his last start; he threw 97. And get this: With that win by Francis, the Rockies were 8-2 with Francis since they signed him June 9, and 5-25 with anybody else getting the ball.

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JAFFE: Six deadline deals that made all the difference

JAFFE: James Shields is on the market. Should he be?

JAFFE: Just how valuable will Ryan Dempster be in a trade?

REITER: Marlins' exec: More moves still possible