Johnson, 26, last year batted .275 with 15 homers and 58 RBIs in 106 games and was unanimously named to the Topps All-Rookie Team ahead of National League Rookie of the Year Ryan Howard. But since opening his sophomore season by going 0-for-27, he's been wildly inconsistent.
After batting .196 with two homers and eight RBIs over the first two months of the season, Johnson bounced back to hit .321 with four home runs and 16 RBIs in June. With July, however, came another major slump; he entered Thursday's opener of a four-game series at Fenway Park batting .132 (5-for-38) in his past 11 games and .237 overall.
And as A's manager Ken Macha noted last week while discussing Bradley's imminent return, "We're getting to the time of year when we need production."
After Thursday's win over the Red Sox, Macha said the decision to demote Johnson came after a series of discussions with general manager Billy Beane.
"We gave him plenty of at-bats to get in that groove, and he never did," Macha explained. "He needs to go down there and get his at-bats every day. That was not gonna happen here."
The subtraction of Johnson and the addition of Bradley, who has been out since June 15 with a strained left shoulder, essentially turns second-year slugger Nick Swisher into Oakland's full-time first baseman for the time being.
"Fine by me," Swisher said. "I like it over there. I don't like to see a teammate get sent down, but Dan's a good player. He'll be back."
Swisher, who led the team in homers (20), RBIs (52), walks (57), total bases (156), runs (63) on-base percentage (.378) and slugging (.510) while tying for the team lead in hits (79) in the first half, made his 22nd start of the season at first base on Thursday and hit his 21st homer. He's started 63 games in left field, where Bobby Kielty and Jay Payton likely will split time, with Swisher at first and Bradley in right.
"I was shocked," Kielty said of hearing about Johnson's fate, "but it happens to everybody."
Bradley went 1-for-4 while playing for the Class A Stockton Ports on Wednesday and finished his rehab assignment 8-for-20 (.400) with two homers and six RBIs in five games with Sacramento and Stockton.
Macha wouldn't speculate as to how long Johnson might be with Sacramento, but he did mention the possibility of a trade resulting in his return.
"You never know what's going to develop," said the skipper. "I told Dan to just go down there with a good attitude and be ready for anything."
Chavez out for series: The A's want to do everything possible to avoid putting third baseman Eric Chavez on the disabled list, so they're giving him the Red Sox series off.
"We don't really know how many more days we'll need," said Larry Davis, the team's athletic trainer. "But this is where we'll start."
Chavez, who has tendinitis in both forearms, didn't have much fun over the All-Star break. He underwent an MRI exam Monday in Phoenix, received treatment in Oakland on Tuesday, and flew with the team to Boston on Wednesday.
"Over the break it felt horrible," Chavez said. "Today it felt good. Some days are good, some days are bad. ... It's confusing."
Macha said he didn't want to play Chavez on Thursday and risk losing him for the next couple of days. Davis said such a cycle would likely continue if Chavez continued to try to play through the pain "if you don't find a way to stop it."
The A's have been slow to place injured players on the DL all season.
"This is probably similar," said assistant general manager David Forst. "Just like we have with a lot of guys, it's gonna be day-to-day with Eric. We're gonna take our clues from him."
Outside counsel: Rich Harden, on the DL with a strained right elbow ligament since June 5, was scheduled to play catch Thursday as the start of his latest throwing program.
Instead he let the A's know -- through his agents at RMG Sports Management, who contacted the front office -- that he'd prefer not to throw until he can get another medical opinion on his ailing wing.
Rather than go to Baltimore with the A's after the Boston series ends Sunday, Harden will head for Los Angeles to be examined by Dr. Lewis Yocum on Monday.
"You never fault a guy for wanting a second opinion," Davis said. "Players have that right, and they should exercise that right whenever they want. ... It's perfectly fine with me. I have no problem with that at all."
Dribblers: The A's have eased up on their unpopular -- with players, anyway -- ban of beer in the clubhouse and on the team plane. Due to liability concerns, the home clubhouse in Oakland will remain dry because players typically drive to and from work, but visiting teams are welcome to provide their own adult beverages, and the A's are once again allowed to partake -- "within reason," a source said -- on the team plane and in the clubhouse while on the road. ... Lefty Joe Kennedy, on the DL since May 11 with left shoulder tendinitis, is scheduled to start a rehab stint with Sacramento on Monday or Tuesday. ... Righty reliever Jay Witasick, on the DL with ankle tendinitis since June 20, struck out two in a perfect inning of work Wednesday with Stockton and is expected to pitch for Sacramento on Friday.
Coming up: All-Star lefty Barry Zito (8-6, 3.29 ERA) takes on Red Sox righty Josh Beckett (11-4, 4.75 ERA) in the second game of the series on Friday. The first pitch is set for 4:05 p.m. PT.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.