"Eric made a long throw during Friday night's game and strained it a little bit," A's manager Bob Geren said before the series finale with the Rays.
Chavez had a scheduled day off Saturday and was examined by a team doctor later in the day.
"He's officially day-to-day," said Geren, who left open the possibility Chavez would be available to pinch-hit.
This is the latest in a long line of setbacks for the 11-year veteran that dates to 2007. After missing most of last season, Chavez reported to Spring Training in good health. He was on his own timetable during the spring but was ready when the season opened.
The A's signed Nomar Garciaparra, in part, to give Chavez a chance to rest.
"We were looking at him and Eric to be one person," Geren said.
Garciaparra is also monitored daily and told Geren he felt fine after getting two hits and driving in three runs in Oakland's 5-2 win over the Rays on Saturday.
Garciaparra, still nursing a calf strain, was in the lineup for the 11th time, taking over for Chavez.
Through 16 games, the two former All-Stars have combined for one home run, seven RBIs and a batting average of .167.
Chavez has played in 121 total games over the past two-plus seasons. He's been shut down by back surgery, right shoulder surgery (twice), microdiscectomy surgery on his back and a left shoulder debridement.
The A's have Chavez under contract through 2010, with a club option for '11. They continue to hope he returns to the form that earned him six consecutive Gold Gloves, and recognition as one of the best sluggers at his position.
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.