After having to leave his most recent start, Parker will try to lead the A's to their eighth win in 10 games Saturday against the Kansas City Royals.
Parker (6-6, 4.11 ERA) is 4-0 with a 2.15 ERA in his last seven starts and had pitched 3 2-3 scoreless innings against St. Louis last Saturday before developing problems with his right hamstring. Oakland lost 7-1 after he left.
"He wanted to try to finish the inning but he still felt it," manager Bob Melvin said. "It's not a prudent thing to do. The unfortunate thing was that might have been as good as stuff as he's had all year."
The A's (51-36) haven't suffered too many defeats lately, however, and continued the momentum from a successful homestand with a 6-3 victory in Friday's series opener. Oakland had dropped seven of 10 before winning six of eight at home.
Coco Crisp drove in three runs in his team's fifth straight win over the Royals (40-43), and Josh Reddick tripled, doubled and had a pair of RBIs. Grant Balfour, meanwhile, tied Hall of Famer Dennis Eckersley's franchise record from 1991-92 by converting his 40th consecutive save and 22nd this season.
"You know, it's an accomplishment. Definitely," Balfour said. "I don't like to look into it too much - I'm superstitious, obviously. But I never knew about this, and it's not something I set out to do. It just happened. I still have a lot of work to do."
The A's will try to help Balfour claim the record while adding to Ervin Santana's winless stretch. Santana (5-5, 2.84) hasn't earned a victory since June 13 despite a 3.32 ERA in his last three games.
The right-hander has more wins over Oakland than any opponent, going 14-6 with a 2.09 ERA in 25 starts, but has lost each of his last two matchups despite a 2.08 ERA in those games.
He allowed two runs and four hits over seven innings May 18 but the Royals fell 2-1 in Oakland. That start is one of 16 in a row in which he's pitched at least six innings, the longest active run in the majors.
He gave up four runs over six frames in Sunday's 9-8 win in Minnesota but also walked a season-high four after posting a 1.56 ERA in his first five starts in June.
"I didn't have my best stuff today," Santana said. "I did not have good command, but I made good pitches and gave my team a chance to win."
The Royals are hoping Alex Gordon can return as soon as possible after learning from the team's medical staff that he did not have a concussion. The outfielder hasn't played since hitting his head on the bullpen fence in Wednesday's 6-5 win over Cleveland.
The team was still awaiting clearance from MLB's independent physician.
"I'm following all the rules and they've cleared me to go, so I'm just going to get back in action and get things going," Gordon said Friday.
Gordon is batting .350 in his last 10 games against Oakland and is 2 for 6 all-time against Parker.