Lee was optioned to Triple-A Gwinnett on Thursday, David O'Brien of The Athletic reports.
Lee made his big-league debut last season but threw just two innings, so he was likely always a long shot to break camp with the team. His excellent 54:6 K:BB and 1.54 ERA in 46.2 innings for Gwinnett last season means he should be one of Atlanta's top options whenever they need another lefty in the bullpen.