Samardzija pitched eight solid innings and Donaldson drove in three runs after sitting out Sunday with a knee injury to lead the Athletics over the Houston Astros 8-2.
"I told him there's no chance he's not starting the day I pitch. So I'll give him a massage on his knee if I have to, to get him in there," Samardzija said, joking.
It didn't come to that, but Donaldson acknowledged after the game that he's still feeling pretty banged up despite returning to the lineup.
"It's not something I think about too much," he said. "It's just one of those things, 126 games now. You learn how to play with stuff."
Samardzija (4-3) allowed six hits and two runs and tied a season high with 10 strikeouts to bounce back from a two-game skid in which he yielded 11 runs combined. The right-hander was 2-7 with the Chicago Cubs before getting traded in early July.
"I took that last one pretty personally and I wanted to come out and have a good one and get back into my groove and how I do things," Samardzija said.
Josh Reddick hit a two-run homer, and Donaldson had three hits. Donaldson doubled twice for his first extra-base hits and RBIs in his past nine games.
Reddick's 10th home run sailed into the seats in right field to push Oakland's lead to 3-0 in the fourth.
Chris Carter hit an opposite-field, two-run homer in the eighth to pull the Astros within one, but the A's added five runs in the ninth to extend the lead to 8-2.
Houston manager Bo Porter was disappointed that his bullpen let a close game get away.
"Getting an opportunity to come up in the ninth inning of a one-run game is completely different than coming up down by six," he said. "The whole object is to keep the game right there."
Scott Feldman (7-10) allowed seven hits and three runs in seven innings.
The Astros couldn't string anything together against Samardzija through the first seven innings. They didn't have more than one hit in an inning until the eighth, when Jose Altuve singled before Carter's homer.
Houston reliever Tony Sipp, who had allowed just 10 walks in his previous 41 innings, walked all four batters he faced in the ninth to push Oakland's lead to 4-2. Donaldson's bases-loaded double off Jose Veras sent two more home, and Derek Norris wrapped up the scoring with a two-run single.
Carter's 31 home runs rank third in the American League, and his 18 homers and 44 RBIs since July 1 lead the majors. Altuve's hit gave him 179 this season, which leads the AL. The pair has carried Houston's offense since the All-Star break while the Astros have dealt with numerous injuries.
Athletics: Manager Bob Melvin said INF Nick Punto (hamstring) is making progress and has been taking grounders. Melvin didn't give a timetable on Punto's return but said he's "starting to see a light at the end of the tunnel with him."
Astros: RHP Brad Peacock, who left his most recent start after five innings because of a sore forearm, will take his scheduled turn Wednesday. Porter said Peacock is fine and was removed mostly as a precaution.
BEHIND THE PLATE
Melvin said he'll probably start new catcher Geovany Soto on Tuesday night and put Norris at DH. Soto was acquired from Texas for cash on Sunday, with C John Jaso on the seven-day concussion disabled list. Soto has appeared in only 10 games this season after starting the year on the disabled list following knee surgery in March before another stint on the DL because of a groin injury.
Oakland RHP Jason Hammel is scheduled to start Tuesday after his previous turn in the rotation was skipped because of two off days last week. Hammel is 1-5 with a 6.75 ERA in seven starts since being traded from the Cubs on July 5.
Houston LHP Dallas Keuchel looks for his first win since July 30.