The A's could have held off on disabling Duchscherer for another couple of days and still back-dated the move to May 15, but the roster move corresponded with the promotion of righty Colby Lewis from Triple-A Sacramento to make his A's debut with his first big-league start since 2004.
"That all weighs into it," Geren said of the timing.
Without Duchscherer and closer Huston Street, who is out with right elbow irritation, Oakland's bullpen is in need of some help; rookie Connor Robertson also was unavailable Tuesday with a swollen right thumb, suffered when he was hit by a batted ball on Monday's series opener.
"We're kicking some ideas around," said Geren, who added that he'll likely use lefty Alan Embree as his closer "more often than not" in the meantime.
Street is eligible to come off the 15-day disabled list May 28.
Kotsay strong in Sac: Center fielder Mark Kotsay, who had surgery on his back on March 8 and is on the 60-day DL, started his rehab stint at Sacramento on Monday and went 1-for-3 with an RBI and a run scored while playing five innings on defense.
Kotsay, who spent last week playing in extended Spring Training games in Arizona, passed every test Monday, including the need to sprint on the bases and wiggle his way out of a rundown between third and home.
"This was probably the most activity I've had in a game so far, as far as running the bases, getting in a rundown and getting back on a ball," Kotsay told the Sacramento Bee. "At this point, everything worked, and there was no pain involved. It'll be interesting to see how I feel when I wake up in the morning, with the amount of work I put in and how I respond."
Geren said Kotsay was scheduled to play five more innings on defense Tuesday, and the plan is for Kotsay to gradually work his way up to playing a full nine innings on defense, with occasional breaks to serve as Sacramento's designated hitter, and be ready to return to the A's lineup May 31. He'll play in a total of 10 games with the River Cats, three at DH.
Righty Esteban Loaiza, who is pitching with torn meniscus in his right knee while he works his way back from a neck injury, was scheduled to start for the River Cats on Tuesday. He, too, is eligible for activation from the 60-day disabled list on May 31.
Dribblers: Rookie outfielder Travis Buck, who left Monday's game with pain in his chronically sore right elbow, was not in the lineup. Buck said it's been sore on and off for the past week and a half, but he has "no idea" what's causing the pain and swelling. "One day, I came in and it was really swollen," he said. "Some days the elbow allows me [to play], and some days it doesn't." Buck added that his wrists, which bothered him throughout the spring and into late April, were "perfect." ... Robertson's thumb did not require X-rays. ... Milton Bradley (hamstring) did not start for the eighth consecutive game but might be able to DH for Oakland this weekend in Baltimore. ... Third baseman Eric Chavez, who was 1-for-13 in his previous three games, was replaced in the starting lineup by Marco Scutaro; Hiram Bocachica started in center field, and Nick Swisher started in right. ... Infielder Donnie Murphy returned from the River Cats' DL -- he had a right shoulder strain -- and went 2-for-4 with a double Monday to boost his batting average to .354.
Up next: A's righty Chad Gaudin (3-1, 2.94 ERA) will get the ball opposite White Sox righty Jon Garland (3-2, 3.18 ERA) in the series finale Wednesday. The first pitch is set for 5:11 p.m. PT.