Vikings defensive end Jared Allen was not happy about the low hit his teammate suffered, calling it a dirty play during a TV interview.
"I feel all right," Williams told reporters after the game. "Don't really know what's wrong, but I think I'm fine. I was just upset that I'm getting cut like that 11 years in from a guy I don't even know who the guy is. Between the pain and the fact that no one was looking it was terrible."
Vikings coach Leslie Frazier said the team may send video of the play to the league office seeking discipline against Niners guard Joe Looney, a second-year player from Wake Forest. Looney was not penalized.
If Williams were to miss time with the injury, it would leave a gaping hole in Minnesota's defensive tackle depth chart. Sharrif Floyd is out for the rest of the preseason after a chop block caused him to have a minor knee operation, and Christian Ballard is still away from the team because of personal issues.
Williams, who will be a free agent in the offseason, restructured his contract in the offseason and will earn a guaranteed base salary of $4.9 million.
ESPN.com's Ben Goessling and Field Yates contributed to this report.