"The doctor on duty looked at it and they decided to get some tests," Showalter said after the game. "They're conducting those tests right now. He had that same issue before."
Earlier this season, Castillo was injured in a similar incident and went on the disabled list with a testicular injury.
"Sometimes you feel better to get away from it," Showalter said. "He was struggling when he came in, so we're going to have to err on the side of faith and see what the test results come back as."
Caleb Joseph replaced Castillo in the second. Catcher Chance Sisco is also on Baltimore's expanded roster.
Keegan Matheson is a reporter for MLB.com based in Toronto. This story was not subject to the approval of Major League Baseball or its clubs.