Pence, batting .243 with three home runs and 16 RBIs, was the only Giant to appear in the club's first 37 games through Friday. But an MRI examination early Sunday revealed a mild hamstring strain.
Pence, 34, expressed optimism about being ready to leave the disabled list when he's eligible May 23.
"Hopefully we can knock this out real quick," said Pence, who expressed satisfaction with this year's change in the length of DL stints from 15 days to 10.
"Fifteen days would be a long time," Pence said. "Ten days is right at what it requires to get this thing right."
Pence missed 48 games with a strained right hamstring last season. He eventually underwent surgery to remove a torn tendon in his leg and returned to play 24 games. However, he pointed out that right-handed batters such as him undergo less stress on the left hamstring than on their right, which they use for pushing off when launching a swing and weight support when they're finishing it.
Pence said he began feeling discomfort in his hamstring last week during the Giants' three-game series in New York.
Williamson, who opened the season on the DL with a strained left quadriceps, hit .274 with three homers and 10 RBIs in 18 games at Sacramento. Giants manager Bruce Bochy said Williamson, who has spent parts of the previous two seasons with the Giants, will be used in both outfield corners. He started in right field and batted eighth in Monday night's series opener against Los Angeles.
Williamson, 26, began Spring Training as a candidate to win the left-field job and hit .324 with a .937 OPS. But his quad injury hampered him, along with the Giants' curiosity in Chris Marrero, who hit seven homers in 64 Cactus League at-bats. They opted to begin the season with him and Jarrett Parker in left.
Marrero quickly faded and Parker was sidelined by a fractured right clavicle in mid-April. But Williamson, who said he lacked proper timing at the plate, remained in Triple-A. A 6-for-13 stretch this past weekend at Colorado Springs propelled Williamson toward his promotion.
"A lot of opportunities present themselves throughout the year in this game," Williamson said.
• Closer Mark Melancon (right pronator strain) threw 22 pitches during a bullpen session. The Giants will evaluate his health before announcing the next step in his recovery Tuesday.