Pence hurt his thumb while diving to make a play on April 3, but he has played through the injury. He's hitting just .172 with one extra-base hit and three RBIs through 17 games this season.
Giants manager Bruce Bochy downplayed the severity of the injury, but offered no timetable for his outfielder's return.
"He needs a few days off and then we'll get him going again," Bochy said. "It's kind of been lingering, so we're going to give him some time off."
Pence hit 13 home runs last season and drove in 67 runs, his most since 2014 when he was an All-Star and finished 11th in NL MVP voting. But his .701 OPS in 2017 was the second worst of his career, and he opened this year with a .386 OPS, although in a small sample size (61 plate appearances).
While nobody was willing to directly correlate the nagging thumb injury with Pence's rough start, adding a hot bat in Williamson made sense with the Giants' offensive struggles.
Williamson, 27, joins the Giants for the first time in 2018 after a hot start to the year with Sacramento. He hit .487 (19-for-39) with six home runs and 16 RBIs in 11 games for the River Cats.
"Hopefully he'll be a shot in the arm for us," Bochy said.