Lopez, who had been out for a month with a right elbow sprain, pitched two scoreless innings against the Giants.
"It was outstanding to see him come back and throw the ball like he did," manager Brad Mills said Sunday. "That was definitely a bright spot."
Lopez threw 29 pitches, so he was not available on Sunday. Mills said having Lopez back in the Houston bullpen helps solidify the entire group.
"Sometimes roles get overplayed a little bit, but when you get an arm like that you can slot into your bullpen. That's pretty nice," Mills said.