He missed Saturday's 2-2 draw with Middlesbrough after breaking his hand training at home during a series of fitness excercises.
And on a day when Fabricio Coloccini retired early with a dead leg and star defender Steven Taylor is already out for another three weeks, Williamson's absence gives Chris Hughton a defensive headache.
Chris Hughton: "He was doing his own fitness excercises at home when it happened.
"He slipped and caught his metacarpal in the back of his hand.
"It was a freak accident, he will be out for a short period of time."