Lyon returned to Houston and had an MRI that showed tendinitis in his biceps and a partial tear in his rotator cuff, which can heal without surgery. The Astros called up right-hander Jose Valdez, who was optioned to Triple-A Oklahoma City on May 2.
Lyon failed to retire any of the five batters he faced during a 3-2 loss to the Cincinnati Reds on Wednesday afternoon, struggling with his control and a subpar fastball. He came into the game trying to finish off a one-hitter but gave up a walk, four hits and three runs.
He worked out and got treatment after the game, avoiding interviews.
A day later, the Astros decided to have an MRI exam on his shoulder, which hasn't felt as strong as usual, manager Brad Mills said. The Astros got their diagnosis after they left the ballpark for Pittsburgh following a 10-4 loss to the Reds.
"He's been getting it worked on and been doing exercises and so forth the last week or so, I should say," Mills said before the game.
Lyon converted 20 of his 22 save chances last season, when he went 6-6 with a 3.12 ERA. This season, he has blown four save chances in eight tries. The loss on Wednesday left him 3-2 with a 7.15 ERA. Lyon has given up 21 hits and nine runs in only 11 1/3 innings.
The velocity on his pitches had diminished, indicating a problem.
"I'm sure it's all tied together," Mills said. "When you get in a situation where the shoulder is a little weak and maybe he doesn't have the strength in the shoulder, he starts compensating in other areas."
Right-hander Mark Melancon will move into the closer's role for now. He leads the majors with 18 appearances and has pitched very well, going 2-1 with a 1.72 ERA. The Astros got him from the New York Yankees last July 31 as part of the deal for Lance Berkman.
Melancon saved 15 games in the minors throughout his career but doesn't have a save in the majors. He blew his only opportunity earlier this season.
"He's been throwing the ball really well," Mills said. "He's got a high level of confidence right now with the way he's been throwing."
The Astros also said that minor league outfielder/first baseman Telvin Nash will have surgery next week on a broken hamate bone in his left hand, sidelining him for six weeks.
The third-round draft pick in 2009 is playing for Class A Lexington. He hurt his hand on a checked swing over the weekend.