"It didn't go as well as I thought it was going to," Willingham said. "I ran on it, maybe 60 to 70 percent, and still felt it. It was one of those things where we had to make a decision on whether or not we were going to play on it, and we didn't think it was going to get any better if I played on it, so we'll just let it heal."
The A's upcoming schedule also played a factor, because after leaving New York, the club will spend the weekend playing in a Philadelphia under National League rules and preferred to have a full bench.
"We were hoping not to do this," manager Bob Melvin said. "Certainly, he's a guy that's very important to us, a guy that hits fourth for us, so we gotta get him right. Taking a chance in trying to run him out here in the next few days was probably not the thing to do."
In the meantime, the A's will get a peek at Carter, who has long been considered the organization's top prospect. During his first Major League callup last season, he endured an 0-for-33 stretch but batted .342 over his final 13 games.
Carter sat on Sacramento's DL from April 22 to June 4 with a sprained left wrist and was 11-for-33 with three home runs and eight RBIs in just nine games upon his return.
The 24-year-old's progress to the big league level has somewhat been delayed by the presence of Daric Barton at first base, Carter's natural position. But Barton's demotion to Triple-A this week following struggles of his own at the plate means that Carter is in line to see time there during his stay.
Willingham's absence could draw Carter a few outfield starts as well, though the A's have several options in Ryan Sweeney, David DeJesus, Conor Jackson and Hideki Matsui. Melvin was admittedly unsure of Carter's role going forward, but Carter appeared at first base as a late-inning subsitution Thursday, making an error in a key moment that allowed Jose Reyes to rap an RBI single.
"One of the things we lack a little bit is some power, and he does have some power," Melvin said. "I know, talking to him, first base is probably his most comfortable position, but we'll see what we figure out going home with the [designated hitter] and the outfield."
Though Carter's promotion signals an exciting time for A's fans, who have been awaiting his return, the move is likely not a permanent one, as Melvin ensured that "certainly when Josh Willingham is back, he's going to be back."
Still, that was seemingly the case when Jemile Weeks was called up to fill in for the injured Mark Ellis, who on Wednesday assumed a utility role upon his return because of Weeks' consistent production. So should Carter offer a similar performance, the A's could consider handing him an extended stay at first base.
Carter, who arrived in New York at 8:45 ET on Thursday morning following a delayed red-eye flight out of Phoenix, did not play the outfield after coming off the DL in Sacramento but is still open to playing the position. At the same time, he said first base "is a natural thing" and helps his focus at the plate.
"There's nothing to press about," he said. "I'm feeling pretty good, seeing the ball well."