But the discomfort returned Tuesday night throughout Frias' five-inning start against the Diamondbacks. Frias is scheduled for a regular between-starts bullpen session Friday.
"I hope it's nothing serious," said Frias, who said he had lower back problems in the Minor Leagues four years ago.
Frias and Mike Bolsinger, who have taken the rotation spots of the injured Hyun-Jin Ryu and Brandon McCarthy, have not pitched beyond the sixth inning since June 9.
Manager Don Mattingly said he plans to shuffle the rotation coming out of the All-Star break to avoid having Bolsinger and Frias back to back, as their shorter starts have taxed the bullpen on consecutive days.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.