Grandal led off San Diego's five-run inning with a walk on Monday night. Batting for the second time in the inning, he had a big swing and miss, then winced and reached for his side. He was replaced by John Baker, who flied out to end the 10-batter inning.
The Padres said Grandal strained a muscle.
The Padres had all four players that they got from the Reds in a trade for starter Mat Latos last December. Right-hander Volquez started the game, first baseman Yonder Alonso and Grandal were in the starting lineup, and reliever Brad Boxberger was called up from the minors before the game.