Duchscherer was scheduled to start against the Los Angeles Angels, but the A's made a last-minute change and sent Tyson Ross to the mound for his first major league start due to a recurrence of the inflammation in Duchscherer's troublesome left hip.
Cust batted .273 with 19 RBI in 33 games with Sacramento and hit four homers, all of them in consecutive games. The outfielder-designated hitter was with the Athletics in spring training before they designated him for assignment on April 3. He then cleared waivers and was outrighted to Sacramento.
Cust led the A's with 25 homers last year, but also led the AL in strikeouts for the third consecutive season.
"We were at three position players, and we thought he could help," manager Bob Geren said. "He was off to a nice start down there and he's swinging the bat well. He just wanted to get his swing and rhythm back, and his eye was outstanding. He had 30-something walks already, so he was swinging at strikes."
Cust became a free agent at the end of last season after the A's declined to offer him a contract for this year, but they eventually re-signed him to a $2.65 million, one-year deal, including performance bonuses that would allow him to make an additional $350,000 if he had 600 plate appearances in 2010.
Duchscherer, who had been sidelined with hip inflammation, lasted just 3 1/3 innings in his last start on April 29, giving up four runs in a 6-3 loss at Toronto.
The two-time All-Star missed last season after he underwent arthroscopic right elbow surgery on March 31 and later was treated for clinical depression.
To make room on the roster for Duchscherer and Cust, Oakland optioned right-hander Henry Rodriguez to Sacramento and designated right-hander Edwar Ramirez for assignment.