The left-hander was hurt while making a lunging catch of a bunt by Diamondbacks pitcher Joe Saunders. The Giants said X-rays were negative.
Arizona scored two runs earlier in the inning to tie the game.
Zito said his foot came down at an odd angle as he made the catch.
"I saw the ball come off the bat and just went to go get it," he said. "I went to push off, I think my right foot was off the ground a little bit, and then I went to push off the jump and that's where it happened. It's really odd. It was just a weird angle and then putting all your weight to one foot to jump."
The 2002 AL Cy Young Award winner, who has struggled since coming to the Giants, was on crutches in the clubhouse after the game with considerable swelling in the middle of his foot. An MRI was planned on Sunday.
Zito, left off the World Series roster last year, has never missed a start due to injury, a fact he said crossed his mind when he was unable to push off while trying a practice pitch after he was hurt.
Zito hurt his neck in a car accident in West Hollywood on the eve of opening day but came back four days later and made his scheduled start.
This time might be more of a challenge, although the Giants have an off day next Thursday.
"When I feel my foot, it's pretty painful. I couldn't throw my pitch too well, but there are incredible things that can happen," Zito said. "I was able to recover from that accident and that was huge, so we'll see. We'll see what the MRI says tomorrow and just go out and take it day by day."
Guillermo Mota relieved Zito and pitched a career-long 4 1/3 innings to get the victory in a 5-3 win.