"We will make a statement later in the week but right now we are trying to focus on beating Bristol City," said manager Nigel Adkins.
Williams' absence means Lee Ridley will keep his place at left-back, having been expected to drop out of the starting XI to accomodate Williams.
Captain Andy Crosby will play after recovering from an eye infection, while new loan signing Kevan Hurst will challenge Ian Morris to play on the left wing.