The trade is pending the review of medical records. Once finalized, the Cubs are expected to get two prospects from the Athletics in return.
Chafin, 31, is having a stellar season. In 43 appearances, he has given up nine runs, including just one home run; and overall, he is 0-2 with a 2.06 ERA and a 0.839 WHIP.
Chafin, though, has struggled recently, failing to record an out while giving up three runs on two hits and two walks in his last outing, on Saturday, against the Arizona Diamondbacks. It snapped a 24⅓-inning scoreless streak for the veteran.
MLB.com first reported that the trade was being finalized.