And limit he did, stopping Harden at 95 after eight shutout innings of two-hit work in a 5-0 victory.
But what if Shane Victorino's looping liner with two out in the fifth inning hadn't fallen safely into right field, and his high chopper in the eighth hadn't been lost in the sun by Harden, giving Victorino another soft single?
Sure, Harden has a history of injuries. He's been on the disabled list six times since the start of the 2005 season, including an April 3-May 11 stint with a strained right shoulder this season.
What if history of a more glorious sort had been possible after Harden notched his career-high 11th strikeout to end the top of the eighth?
"He might go back [out for the ninth] for a no-hitter, yeah," Geren said after the finale of a three-game Interleague series at McAfee Coliseum.
"No, he would have," Geren corrected himself. "Definitely. Yes. How's that?"
When Victorino's first hit fell in, the home crowd gave their man a nice little ovation, and Harden admitted that he was aware it was the first hit of the day against him. But having reached the seven-inning mark just twice in his 10 starts this season, he was more concerned with staying in the game as long as possible.
"That's been the goal, to get deeper into games and be more efficient," he said. "It's not like [the first hit] was in the eighth or ninth inning."
No-hitter, one-hitter, two-hitter, whatever. Harden was just plain dominant, and he's been dominant for most of the year. The victory improved his record to 5-0 with a 2.15 ERA overall, 3-0 with a 1.79 ERA in eight starts at home, and 3-0 with a 0.68 ERA in four Interleague outings.
"He has a fastball that touches 97 [mph], and then you have a changeup that acts like a split, two-seamer or a cutter," Phillies third baseman Greg Dobbs said. "When you have those pitches, you're going to have some success. He got ahead, humped it up when he had to, got us to foul it off.
Phillies manager Charlie Manuel, whose powerful but slumping-of-late club has been shut out five times this year, was similarly impressed. "That might have been the best game pitched against us," he said.
Geren said he couldn't think of a more dominant outing by Harden, whose name is sure to start to popping up in trade rumors should the A's fall out of playoff contention -- as long as he's healthy and thriving.
Oakland has won eight of its past 12 games and pulled to within 4 1/2 games of the division-leading Angels, who were idle Thursday, in the American League West.
"When he's on," Geren said of Harden, "he's almost unhittable."
The A's jumped on Phillies starter Adam Eaton right away, with Mark Ellis leading off the bottom of the first inning with a walk before scoring on a double by Ryan Sweeney, who in turn scored on an RBI single by Jack Cust. Sweeney sprained his left ankle while sliding into home and left the game before the start of the third inning.
Oakland padded the lead in the sixth on rookie outfielder Carlos Gonzalez's second homer of the year, a solo shot to right field off Eaton (2-6).
Harden struck out five of the first nine batters he faced and didn't allow a second baserunner until Carlos Ruiz drew a walk for the Phils in the sixth. Victorino's second hit gave Harden a new appreciation for the challenges his infielders and outfielders face during day games.
"I didn't know it was that bad," said Harden, who turned away from the ball as it came down to earth. "It's tough out there. I completely lost it."
Victorino stole second base with two out, but Harden finished with a flourish by fanning Ruiz to earn a standing ovation as he ended his day.
"They have a really tough lineup," Harden said of the Phillies, who feature two former MVPs (Jimmy Rollins, Ryan Howard) and a legitimate 2008 MVP candidate in Chase Utley. "But you try to pitch to your strengths instead of their weaknesses."
When A's lefty Alan Embree jogged out of the bullpen for the top of the ninth inning, the announced crowd of 17,228 made their displeasure known with a lusty chorus of boos.
"Were they booing?" Geren mused. "I didn't even hear it."
Geren said Harden didn't lobby to stay in the game, and the pitcher confirmed as much, noting that the A's were up 3-0 when the decision was made to lift him. Oakland tacked on a pair of insurance runs off Tom Gordon in the top of the eighth.
"There wasn't much discussion," Harden said. "I was fine with it. It was a save situation for [closer Huston] Street [at the time], so why not go to him? No sense in pushing it just for the complete game."
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.Less