"[He] still has some symptoms of the illness," Farrell said before Friday's game. "He's unavailable today."
A virus has been circulating through the clubhouse over the past few weeks, and Farrell said that the team wondered if the cause could have been the very high humidity in Baltimore.
Napoli will undergo tests to try to solve the puzzle.
Jeff Seidel is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.