"Realistically, we're probably looking at Saturday or Sunday," Chavez said after Oakland's 12-inning victory over the Red Sox at Oakland-Alameda County Coliseum.
Chavez on Monday said he didn't think the pain he's feeling is related to the surgery that repaired his torn labrum late last summer; it was in his neck as much as in his shoulder.
"Eric doesn't think there's anything major going on," Geren said, "but it certainly doesn't hurt to take a look, just to be sure."
Chavez said he was told Tuesday that there is no structural damage and that the pain in his shoulder is likely tendinitis.
"My neck feels a lot better today," he added. "We just need to calm the shoulder down a little now."
With Chavez out, Nomar Garciaparra started at third base for the second night in a row and left with tightness in his right calf after one inning, but Garciaparra said he removed himself from the game as nothing more than a precautionary measure.
Geren didn't want to discuss any hypotheticals related to Chavez being out for an extended period of time, but what he did say on the topic suggested that Garciaparra, who homered against his former team Monday, will be the primary fill-in at the hot corner.
Mindful that Garciaparra, 35, missed 90 games with various injuries last year and has spent time on the disabled list 12 times in his career, the A's are wary over overusing him.
"Nomar's played extremely well wherever I've put him," Geren said. "I'd like to put him out there as much as possible ... within reason."
When Garciaparra needs a break, Bobby Crosby will take over at third; Geren said he expects Crosby to start at third in the series finale Wednesday. With Jason Giambi, 38, needing regular rest, Crosby also figures to see plenty of time at first base while Chavez is out.
"If Eric's out and Jason needs a day off, or we want to use him at [designated hitter], it'll be Nomar at third and Bobby at first," Geren said. "Bobby's fine over at third, but Nomar's more comfortable there because he's done it more."
Garciaparra was Boston's starting shortstop for 8 1/2 years before being traded to the Cubs in 2004, but Crosby, who lost his job as Oakland's starting shortstop when Orlando Cabrera was signed this spring, is Oakland's main backup for Cabrera and second baseman Mark Ellis.
Ellis, who also had surgery on his right shoulder late last season, hasn't suffered any setbacks and was back in the starting lineup Tuesday after getting most of Monday off; he came on late in the game when Geren lifted Giambi and moved Crosby from second base to first.
Cabrera, 34, hasn't missed a start this season, and Geren didn't sound very concerned about Crosby's shortstop skills getting rusty.
"At the moment, not really," said the skipper. "[Cabrera] played 161 games last year [with the White Sox]. He's pretty good at taking care of himself and pacing himself. I don't see that changing any time soon."