Manager Brad Mills said Lopez doesn't have as much movement on his ball and was taking longer to get loose. Lopez, who has a 7.71 ERA in six appearances, was unavailable to pitch Thursday because of his recent workload.
"We'll get him looked at and respond accordingly," general manager Ed Wade said. "Hopefully it's nothing more than it was before, but it behooves us to take advantage of a really good medical team to find out."
Lopez went 5-2 with a 2.96 ERA in 68 appearances for the Astros last year and came into this season as the setup man. He walked only five batters last year and struck out 50, ranking third among relief pitchers in the Majors in strikeout ratio.
"The guy was pretty automatic all last season for us, and the eighth inning was in good hands as far as we were concerned," Wade said.