The A's righty leads the Major League's with an ERA of 1.64, sitting slightly ahead of San Diego's Jake Peavy, who has a 1.68 ERA.
Haren has allowed 15 earned runs this year in 12 starts and has not allowed over three runs in a game. In fact, he has done that only once, allowing two runs four times and allowing one run or less the other seven times.
He has also won his last six decisions after losing his first two, despite allowing one earned run combined in the two starts.
Haren will be tested as he faces a Boston Red Sox team that ranks third in all of baseball in team batting average (.278) and runs scored (285).
"They're a good team, just look at their offense," A's manager Bob Geren said. "They're just good all the way around."
Haren picked up his only career win against the Red Sox in his last start against them. That start came on July 26 of last season, when he went seven innings and allowed one run.
He will need to be careful with Red Sox outfielders Manny Ramirez and Coco Crisp. Ramirez has five hits in nine at-bats and Crisp has five hits in 12 at-bats.
He has handled David Ortiz pretty well, not allowing a home run to the slugger, who has only three hits in 11 at-bats against Haren.
OAK: RHP Dan Haren (6-2, 1.64 ERA)
Haren is 1-3 with a 3.57 ERA in four lifetime starts against Boston. He also faced them twice in relief in the 2004 World Series as a member of the St. Louis Cardinals and threw 5 2/3 scoreless innings.
BOS: RHP Julian Tavarez (3-4, 5.40 ERA)
Tavarez has spent most of his career as a reliever and has never made a start against the A's. He has appeared in 20 games against Oakland over his career, going 1-1 with an ERA of 4.85.
Player to watch
Mark Kotsay has five hits in 11 at-bats against Tavarez, good for a .455 career batting average. He has a double and a pair of RBIs off the righty.
FSN Bay Area.
Tuesday: Red Sox (Daisuke Matsuzaka, 7-3, 4.83 ERA) at A's (Lenny DiNardo, 1-2, 1.80), 7:05 p.m. PT
Wednesday: Red Sox (Tim Wakefield, 5-6, 4.24) at A's (Joe Kennedy, 1-4, 3.30 ERA), 7:05 p.m. PT
Thursday: Red Sox (Curt Schilling, 5-2, 3.91 ERA) at A's (Joe Blanton, 5-3, 3.81), 12:35 p.m. PT
Matt Smith is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.