Lyon, who blew his fourth save in eight tries Wednesday, returned to Houston on Thursday afternoon to undergo an MRI and be examined by team medical director Dr. David Lintner. Lyon has been experiencing weakness in his right shoulder.
"We've had some questions about whether the arm strength in there has been exactly what it needs to be," said Astros manager Brad Mills, who has appointed right-hander Mark Melancon as the closer in Lyon's absence.
Lyon's velocity has been down, and he allowed all five batters he faced in the ninth inning Wednesday to reach, letting the Reds to rally for a 3-2 win in the ninth. Lyon didn't speak to reporters after the game, but the team announced early Thursday he was going to Houston.
"He's been on the [daily medical] treatment sheet with a little bit of a biceps tendinitis type of thing, but the more they talked yesterday Brandon acknowledged he had some weakness in his shoulder so we just felt it behooved us to get him back into Houston and see Dr. Lintner and try to determine what the best course of action will be," Astros general manager Ed Wade said.
Lyon had a small cyst drained in his shoulder in January, 2010, but it's not known if his current shoulder problem is related.
"We don't need him to try to be heroic at this point in time and fight through something that won't resolve without rest," Wade said. "If rest is the proper course of action, it makes sense for us to give him that opportunity. We saw it last year what he did for us the second half of the season. He had 19 saves [after Aug. 1], one of the best in baseball in the second half. We need that type of effectiveness, but at the same time we need to give him the opportunity to get to where he needs to be."
Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.