"It was really painful at the start of the game and with the intensity of the soreness, we had to take him out," Padres manager Bud Black said. "The pain got worse the more he was on it."
Black indicated that Forsythe -- the leading candidate in camp to replace injured third baseman Chase Headley on Opening Day -- wouldn't "play in a couple of days."
The Padres open the season in six days at Citi Field against the Mets.
"It looks like it's headed that way," Black said of Forsythe going on the disabled list.
Forsythe was unavailable for comment after a 4-2 loss to the Giants.
Forsythe, who was 0-for-2, was replaced in the field by Gregorio Petit. Petit and Cody Ransom are the two players who have received the most playing time at third base.
Petit and Ransom are both in camp on Minor League contracts, which means the Padres would have to add one of them to the 40-man roster before Opening Day. That means dropping someone off the 40-man, which isn't always an easy proposition.
San Diego general manager Josh Byrnes said the team is unlikely to go outside the organization to find a third baseman.
Headley, who led the National League in RBIs last season (115), fractured the tip of his left thumb in a game March 17. He's expected to miss most of the first month of the season.
Infielder Jedd Gyorko, who played third base as a Minor Leaguer, is expected to be the team's Opening Day second baseman. The Padres could, conceivably, move Gyorko to third base and allow utility man Alexi Amarista -- who is hitting .328 this spring -- to play second base.
"We have some options with guys in camp," Black said.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.