Gallardo returned Saturday to Baltimore to be examined by Dr. Michael Jacobs, the club's orthopedist, and will have a MRI Sunday to detect the severity of the injury.
"Until we get the MRI and Dr. Jacobs sees it, you don't know exactly what it is," Orioles manager Buck Showalter said Saturday. "I don't think anybody knows what's causing the symptoms he has. It's not acute pain or anything. It was just general weakness there."
Gallardo complained of a sore shoulder in his start Friday against the Kansas City Royals and was pulled after two innings and 45 pitches.
"My last start in Texas, that was the best stuff that I had since the season started," Gallardo said after the 4-2 loss Friday. "I was getting into the seventh inning and then something like this. It's definitely frustrating. The most important thing is to stay positive and see what happens. Hopefully it's nothing serious."
Gallardo has made 30-plus starts the past seven years, but the Orioles were concerned about his shoulder after a February physical. They signed him but reconstructed his contract, cutting their original offer from three years to two with an option. Gallardo, 30, is guaranteed $22 million over two years, including a $2 million buyout for 2018.
Gallardo has been on the disabled four times in his career, but never for an arm issue. The last time was 2010 with an oblique strain. He has won 12 or more games in six of the past seven seasons.
The Orioles activated left-hander Brian Matusz, who had been on the disabled list with a left side strain. Matusz will be used as a reliever.
Right-hander Kevin Gausman, who has been on the disabled list with a right shoulder strain, will be activated Monday and start at Tampa Bay. He will take Gallardo's place in the rotation.