The switch-hitting Callaspo, who was out the minimum 15 days with a strained right hamstring, was not in Sunday's starting lineup but could potentially see time at first base during the upcoming series in Houston, where the Astros are starting a pair of lefties.
Callaspo's ability to handle the position was a big factor in the A's decision to send down Freiman and keep second baseman Eric Sogard, who is digging his way out of an offensive slump in which he has six hits in his last 16 at-bats.
Freiman hit .212 with one homer -- a walk-off shot in Miami -- and six RBIs in 14 games, including eight starts.
"Callaspo's able to play some first base, so unfortunately for Fry, he was the guy," said manager Bob Melvin. "But he always does a nice job for us, is always prepared, a very hard-working kid who was very impactful for us last year. I wouldn't be surprised to see him back here at some point."
Callaspo will continue to play some second base, too, with Melvin deeming his decisions at the position "a day-to-day thing."
"We always try to run our best lineup out there on a particular day, whether it's matchups, someone that's hot," he said. "We try to do the best we can."