Lopez, 35, started 33 games for the Arizona Diamondbacks last season and had a 7-16 record with a 5.00 ERA.
The Braves signed him to a minor league deal before the season. Lopez is 6-1 with a 2.59 ERA for Triple-A Gwinnett.
Lopez will be added to the Cubs' roster on Friday.
Bruce Levine covers baseball for ESPNChicago.com and ESPN 1000.