The moves were made before Monday night's game against the Los Angeles Angels.
Boston manager John Farrell said Bradley Jr. was sent down to work on changes in his swing. The 24-year old was hitting just .216 with one homer and 30 RBIs in 112 games.
The 21-year old Betts, a second baseman in the club's system at the start of the season, is back in the majors for the third time this year. He moved to the outfield at Pawtucket before hitting .244 with a homer and two RBIs over 13 games in his previous two stints with the Red Sox.