They hope their All-Star third baseman will return Monday night when they visit the Houston Astros.
Oakland (76-53) had moved back into a tie for first place with the Los Angeles Angels after taking the first two of a three-game set, but Scott Kazmir was shelled for seven runs in three-plus innings of a 9-4 loss on Sunday, letting Los Angeles leave town with a one-game edge.
"It's not even about a series any more," manager Bob Melvin said. "We have to bear down every single day. When you take the first two, you always want the third one, but you can't complain when you take two of three."
Donaldson sat out the finale after being bothered by his left knee during an at-bat Saturday and is the latest among a rash of injuries for the Athletics. Melvin said on Sunday night's broadcast that an MRI showed nothing structurally wrong, though, and he's considered day to day.
The slugger has hit .362 with five homers and 15 RBIs in the season series against Houston, which Oakland leads 8-5.
The Athletics also placed closer Sean Doolittle on the 15-day disabled list Sunday with a strained right intercostal. Doolittle has converted 20 of 23 save chances with a 2.28 ERA in 54 games. Catcher John Jaso is expected to join Doolittle on the sidelines Monday because of concussion symptoms.
Jeff Samardzija (3-3, 4.07 ERA) hasn't been as good since he was acquired from the Chicago Cubs, for whom he was 2-7 with a 2.83 ERA in 17 starts. He suffered his worst outing yet with Oakland on Wednesday, surrendering seven runs over 3 2-3 innings of an 8-5 loss to the New York Mets.
"We have a lot of confidence in him and he's pitching some really good games for us," Melvin told MLB's official website. "Today was really the one tough game that he's had overall as far as his numbers go."
Samardzija made two starts against Houston (55-76) last month, limiting the Astros to one run in eight innings of a 13-1 victory on July 24, then giving up four runs over 6 2-3 innings without taking a decision in a 7-4 road victory five days later. He's 3-1 with a 2.86 ERA in five career starts against them.
Scott Feldman (7-9, 4.37) is scheduled to pitch for Houston, but he reportedly was placed on revocable waivers Sunday and could be claimed by another team. He limited the New York Yankees to two runs in 6 2-3 innings in a 5-2 victory on Wednesday, though his 121 pitches matched a career high.
Feldman faced Samardzija both times last month, giving up six runs and walking a season-high five over 5 1-3 innings on July 24. He was much better in the rematch, yielding one run in seven innings, though he still owns a 5-7 record and 6.55 ERA in 15 career starts against them.
Coco Crisp is just 3 for 36 over his last nine games against the Astros, but he's 5 for 12 versus Feldman. Donaldson is 1 for 8 with three strikeouts against the right-hander.
Houston lost 3-1 at Cleveland on Sunday and has managed just eight runs and 19 hits while losing three of four.
Jason Castro is 5 for 12 off Samardzija with three doubles.