Forsythe, who was to compete with Everth Cabrera and Andy Parrino for an infield job, was told that he suffered a fracture of the sesamoid bone.
San Diego manager Bud Black said Forsythe will miss a minimum of two weeks. Forsythe said the team won't opt for surgery right now, though it remains a possibility.
"This week, we're going to take it easy and, with treatment, see if we can get it to calm down," Forsythe said. "We'll go from there."
Forsythe said the fracture to the sesamoid bone -- which helps with weight bearing and elevates the big toe -- wasn't due to a specific incident. In fact, he had the same fracture to the bone in his right foot in college at the University of Arkansas.
In that case, Forsythe said, he had surgery and missed between "two and three months."
"I think it just has to do with the baseball activity, running around on the field, wearing cleats," Forsythe said. "You can play on it, but it alters the way I run and do my things on the field. It can hurt other things like my knee and hips.
"It comes from overwork, constant pounding on the foot."
Forsythe admitted to being frustrated Saturday but is hoping the foot won't require surgery and that he can return to the field this spring.
"It's kind of day by day," he said. "It's frustrating. I put in a lot of work this offseason and was really excited about this camp. It's tough. We'll see what happens."
Also Saturday, Black said that shortstop Jason Bartlett will miss a few days with a sore right knee. Black said the team is being cautious with Bartlett because it's so early in camp.