Ziegler did make it to Rangers Ballpark in Arlington around game time Thursday. Geren said his symptoms -- a "non-baseball-related" condition usually means an illness -- were not serious, but he was unavailable to pitch.
The A's were also without first baseman-designated hitter Jason Giambi, who is having trouble with both hamstrings, for the first six innings Thursday. Giambi came in to pinch-hit in the seventh, popping out to end the inning. When Giambi returns to the starting lineup, which could be as early as Friday in Seattle, he could be used as the DH.
"Jason is going to need another day," Geren said. "We're optimistic he'll play [Friday]. He had treatment, and he had some improvement overnight."
There was no update on reliever Santiago Casilla, who is dealing with a sore calf.
The A's, off to a somewhat slow start, are already dealing with five players on the disabled list, having placed Nomar Garciaparra and Mark Ellis on the 15-day DL on Wednesday.
Ellis returned to Oakland on Wednesday.
The A's began the season with three players on the DL -- starter Justin Duchscherer, potential closer Joey Devine and outfielder Ben Copeland.
Duchscherer, who was placed on the 60-day DL on Wednesday, has begun playing catch and could pitch in a game in a month.
Devine is out for the year after season-ending Tommy John elbow reconstruction surgery, and Copeland is out with a sprained left shoulder.
Todd Wills is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.