So far, Haren's title chances seem much better than Santana's.
Playing for the best team in the majors, Haren looks to help the Diamondbacks win a series at home against the Mets for the first time since 2003 as the teams conclude a three-game set Sunday.
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There was plenty of interest in Haren and Santana from teams that felt they could be top contenders this season, and the two aces were eventually traded to Arizona (21-9) and New York (15-13), respectively.
The Diamondbacks acquired Haren (4-1, 3.13 ERA) from Oakland for six prospects last December, pairing him with Brandon Webb to give them a potent 1-2 combination at the top of the rotation.
"He was the guy we targeted early on in the process," Arizona general manager Josh Byrnes told the team's official Web site. "He was attractive for a number of reasons."
The Mets, meanwhile, were able to pry Santana (3-2, 3.12) away from Minnesota in February in exchange for four players.
Haren and Santana are two of just four pitchers to throw at least 200 innings with an ERA below 3.50 in each of the past two seasons - Atlanta's John Smoltz and Houston's Roy Oswalt are the others - and their additions appeared to make Arizona and New York two of the favorites to win the National League pennant.
While the Diamondbacks have lived up to expectations, the Mets have been inconsistent, largely due to injuries and poor relief pitching.
Webb became the majors' first seven-game winner Saturday, leading Arizona to a 10-4 win in the middle game of this series. A victory Sunday would give the Diamondbacks a series win at home against the Mets for the first time since taking two of three from Aug. 8-10, 2003.
Haren won his last start, allowing three runs and five hits while striking out five in 7 2-3 innings as Arizona beat Houston 5-3 on Monday night. The right-hander has a 2.79 ERA and has not received a decision in one start and one relief appearance in his career against the Mets, but he hasn't faced them since 2005.
Santana is 2-0 with a 0.56 ERA in two career starts against Arizona. He hasn't faced the Diamondbacks since June 8, 2005, when he struck out nine en route to a four-hitter in his only career Chase Field appearance, a 10-0 Twins win.
Santana has given up two runs or less in four of his six starts this season. The left-hander allowed two runs and two hits while striking out seven in 5 2-3 innings against Pittsburgh on Tuesday, but did not receive a decision as New York won 5-4 in 11 innings.
Santana will be facing an Arizona lineup coming off one of its best games. Augie Ojeda, starting in place of injured second baseman Orlando Hudson, drove in a career-high six runs as the Diamondbacks matched a season high with 15 hits.
Hudson, who sat out with a mild hamstring injury, is expected to return Sunday.
Mets left fielder Moises Alou, meanwhile, got a game off after returning from the disabled list and playing his first game of the season Friday night in a 7-2 win over Arizona. Alou, who had been on the DL since March 23 following hernia surgery, should be back in the lineup Sunday.
Alou is 8-for-14 (.571) with two home runs and two doubles against Haren.