Gallardo, who flew home to Baltimore on Saturday, will be examined by Dr. Michael Jacobs and undergo an MRI on Sunday. The team should know more about his condition after that.
"Nobody really knows. You have to put something on the sheet. The team doctor for [the Royals] saw him," manager Buck Showalter said of Gallardo's initial diagnosis. "Until we get the MRI and Dr. Jacobs sees it, you don't know exactly what it is.
"I don't think anybody knows exactly what is causing the symptoms he has. It wasn't like he was feeling acute pain or anything. There was general weakness there."
Gallardo, a 10-year veteran, had never been on the DL for an arm-related issue and has made at least 30 starts in seven consecutive seasons. But the 30-year-old felt something in his shoulder warming up and lasted just two innings on Friday.
"My shoulder just didn't feel right. To be honest, it was one of those feelings that I've never had my whole career," Gallardo said after the 45-pitch outing. "I've never had any issues with my shoulder, and I hadn't been [having any]. For some reason, in the bullpen it just didn't feel right. I wasn't able to extend to get that release point. I think that's why the ball was just up and all over the place. Felt like the ball was coming out of my hand and the strength wasn't there."
Gallardo's shoulder was a concern this spring, so much so that the Orioles -- who are well-known for stringent physicals -- dropped their initial offer from three to two guaranteed years with a club option for 2018. Gallardo's velocity was down last season, sparking some concern at the time of the signing, though Showalter felt as if the righty was headed in the right direction after coming off of his solid start in Texas.
"He's a couple ticks below [velocity-wise], but a lot of guys, as they get on in their career, start off that way and pick up as the season goes on," Showalter said on Friday night. "He touched 90 [mph] two or three times the last time out in Texas, so we were hoping it would be a little bit better this time, but it wasn't.
"I've done all the looks at his average velocity in April and May. Every start I've looked at them, he's down."
Matusz, 29, was placed on the 15-day disabled list on April 3 (retroactive to March 25) with a left intercostal strain.
Matusz made five rehab appearances (one start) between Class A Advanced Frederick and Double-A Bowie, recording 16 strikeouts in 10 innings. In 272 career Major League games (68 starts), Matusz is 27-41 with a 4.76 ERA.