Cornely got all of one inning in his cup of coffee with the Braves, and was crushed for three hits, including a homer, and a walk against the Nationals back on April 29. The outing came in garbage time, with the Braves already down 9-4, so it wasn't a high leverage situation.
Cornely was removed from the 40-man roster as a result of the Nick Masset acquisition, and the Braves are getting the always vague "cash considerations" as a return. I always wonder if the "consideration" is allowed to be "we considered it, and nope." Or would that be uncouth?
The Red Sox have a bunch of 40-man roster spots, so it makes sense that they'd roll the dice on a guy with Cornely's live arm. They probably hope Cornely sticks around longer with them than acquisition Anthony Varvaro, who was traded from the Braves to the Sawx earlier this season and was eventually DFAed despite pitching decently.
Cornely was a 15th-round pick back in 2011 by the Braves who turned some heads in Spring Training after a pretty good 2014 in AA (2.5ish ERA, 3ish FIP, striking out a fourth of all hitters faced) and I wish him the best of luck.