Lopez, who's among the most used relievers in the National League, went on the 15-day disabled list Friday with a sprained right elbow. He flew to Houston from San Francisco and was examined Thursday by team doctors, who determined he needed some rest.
Houston activated left-hander Fernando Abad, giving manager Brad Mills three lefties in the bullpen.
The workhorse Lopez was 3-0 with a 2.51 ERA in 32 games for the Astros.
"It was really unfortunate," Mills said. "He threw Sunday [in Chicago] and felt fine and came out Tuesday and felt a little tenderness. After getting him checked out, we felt it would be best to give him some rest."
Abad had been on the DL since May 26 with a right intercostal strain. He'll join Xavier Cedeno and Wesley Wright as the lefties in an eight-man bullpen.
"We want to get him back to like he was throwing the ball earlier in the year," Mills said.