They have two of the oldest and most experienced rosters in baseball. One team has three regular position players under the age of 30; the other has just one.
But where would the Red Sox and Yankees be this fall without the help of their late-season call-ups?
In New York, Joba Chamberlain has stabilized the back end of the Yankees' bullpen, providing a critical link to closer Mariano Rivera. Starters Phil Hughes and Ian Kennedy have stepped in for the ineffective Mike Mussina and the sidelined Roger Clemens, helping the Yanks virtually clinch the American League wild card.
The Red Sox, meanwhile, have benefited from the energy and play of outfielder Jacoby Ellsbury, whose playing time has increased with the loss of Manny Ramirez (strained oblique). Earlier this month, when the Sox needed an extra starter thanks to an injury to Tim Wakefield, Clay Buchholz stepped in and tossed a no-hitter in his second major league start.
As the Red Sox and Yankees prepare to meet for their final series of the 2007 season, they are changed -- for the better -- by the introduction of their rookies.
"Obviously, they've helped and brought some energy," said an AL executive with another club. "But what they've done more than most [call-ups] is they've stepped in and done the job. They've come up and made an immediate impact. There's been a seamless transition. When they come up and contribute, it's like a breath of fresh air."
Not long ago, when rosters expanded in September, only teams far out of contention counted on their rookie call-ups. The final month was an opportunity to evaluate young players, introduce them to the major league culture and provide a head start for the following year.
But now, teams -- and times -- have changed, even for big payroll teams like the Red Sox and Yankees.
"I think K-Rod [the Angels' Francisco Rodriguez] came up and performed so well [in 2002], it opened a lot of eyes and changed the way people think," Red Sox general manager Theo Epstein said.
Deadline deals have become more difficult to make, since potential trading partners inevitably ask for those same can't-miss prospects teams so carefully protect and nurture. Reluctant to part with their homegrown players, teams often turn to those same prospects and hope for immediate contributions.
"When young guys come in and contribute, I think it does spark a team a little bit," Epstein said. "The grind of the season can become monotonous, and when you have 24 guys rooting for a new player, it builds camaraderie and sharpens the focus. And when it happens in dramatic fashion, as it has with our guys and with Chamberlain, it can serve as a bit of a catalyst."
Moreover, for teams like the Yankees that have been forced to play catch-up for much of the season, exhaustion sets in as the final month of the year dawns.
"You expend all this energy trying to make up ground," one scout said. "Then, you get there, and especially with an older team like the Yankees, there's not a lot of gas left in the tank."
For the better part of the past two seasons, the Yankees searched for a reliable bridge to get them from the middle of the game to their unparalleled closer, Rivera. They settled on Chamberlain, a converted starter, whose introduction to the big leagues might have been patterned after Jonathan Papelbon's in 2005.
Chamberlain immediately took to the new role and was unscored upon in his first 11 appearances, until the Toronto Blue Jays nicked him for an unearned run Wednesday night. In 12 appearances, covering 16 innings, he has struck out 20 while walking five and holding opposing batters to a .143 batting average.
"He comes right at you with two quality pitches," one scout said, "and his stuff is good enough that he can get away with some mistakes."
Kennedy, 22, stepped in and took over Mike Mussina's spot when the veteran stumbled badly in August. The Yankees won both of Kennedy's first two starts as he allowed just three earned runs in 12 innings. After Thursday's start at Toronto, a no-decision for Kennedy and a loss for the Yankees, he is 1-0 with a 1.89 ERA.
The Yanks had turned to Hughes earlier in the year when injuries decimated their staff, only to have Hughes himself lost to the disabled list with a severe hamstring pull. But Hughes has returned to the rotation, and the Yankees have won both of his September starts.
In Boston, Ellsbury, the Red Sox's first selection in the 2005 draft, has been compared to Johnny Damon. Like Damon, Ellsbury has a slashing swing, well-above-average speed and rangy athleticism in the outfield.
In a dozen September games, Ellsbury has shown surprising power, launching three homers -- one more than he hit in 436 minor league at-bats this year. Ellsbury attributes this sudden show of power to an adjustment he made on his own in the minors -- after the second of his first two (brief) promotions to Boston.
"He's been terrific for us," manager Terry Francona said. "I can't say enough about what he's able to give us [with Ramirez out]."
Buchholz, by contrast, has been something of a shooting star -- briefly brilliant, then out of sight. After no-hitting the Baltimore Orioles on Sept. 1, he has made just one other appearance -- providing three scoreless innings in relief -- as the Red Sox look to carefully manage his innings and workload.
It's possible he could be the team's secret weapon in October if Daisuke Matsuzaka and Wakefield fail to rebound from recent downturns.
It's worth noting that every one of these phenoms, except Kennedy, had exposure to the big leagues before Sept. 1.
"Give credit to both teams," a rival general manager said. "They weren't looking to catch September magic. They systematically thought this out."
And now, in the heat of the pennant race, they are reaping the benefits.
Sean McAdam of The Providence (R.I.) Journal covers baseball for ESPN.com.