"It's just general soreness," Red Sox manager John Farrell said after the Red Sox beat the Giants 12-1. "He's played first base two of the last three days and he's not used to it."
Ortiz was hitless in three at-bats in Boston's win on Wednesday, striking out his last two times at the plate. He came out of the game following the top of the fifth inning and was replaced by Mike Napoli.
"With the lead I thought we could get him off his feet and protect him," Farrell said.
Ortiz entered the day batting .323 with 24 home runs and 78 RBIs.
The Red Sox have a day off before opening a series at the Los Angeles Dodgers on Friday night.