Willis suffered the knock during Monday night's 31-17 win over the St Louis Rams and he was seen wearing a protective boot as he left the locker room.
"We'll find out more [Tuesday]." Willis told reporters as he limped to the team bus.
Although X-rays were taken after the game they didn't show any significant damage, but the injury was serious enough to ensure that Willis did not return to the field after leaving in the second quarter.
The 29-year-old has 34 tackles so far this season and he is 50 short of 1,000 in his career ahead of the clash with the Denver Broncos on Sunday night.