The A's sent reliever Ryan Cook to the Boston Red Sox just before the trade deadline for a player to be named later or cash, a source confirmed to CSNCalifornia.com.
Cook, 28, pitched four games for the A's this year before he was optioned to Triple-A on May 3.
After going 0-2 with a 10.38 ERA in the majors, Cook was 4-1 with a 4.05 ERA in 30 Triple-A appearances with the Nashville Sounds.
An All-Star in 2012, Cook is 13-12 with a 2.93 ERA over five MLB seasons, four with the A's.
Jon Heyman of CBSSports.com was the first to report the deal.