Arias, 25, is 2-1 with a 3.35 ERA in 42 relief outings for Houston this season, having made the most appearances by an Astros right-handed rookie pitcher since Chad Qualls appeared in 77 games in 2005. He made 22 consecutive outings without allowing an earned run from May 21-July 24, including a 21 1/3 consecutive-scoreless-innings streak over 17 outings from May 27-July 18.
Lopez, 26, was 4-5 with a 4.73 ERA in 29 games, including 15 starts, for Corpus Christi this season. In his five starts in August, Lopez has a 1.23 ERA with a 2-2 record. Lopez was claimed by the Astros off waivers from the San Diego Padres on April 10.
Lopez has spent four seasons in the Minor Leagues with the Astros (2009), Padres (2007-08) and Yankees (2004). He was originally signed by the Yankees as a non-drafted free agent in 2002.
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.