Williamson is struggling with a hamstring problem and striker Henderson has pulled a gluteal muscle.
Kevin Blackwell said: “If either player is back in contention before the end of January it would be an unexpected bonus.”
But Ched Evans (ear infection) and Andy Taylor (virus) could return at Leicester.
The Blades, two points behind Leicester, are hoping to leapfrog the Foxes into the play-off places by extending their unbeaten league run to seven matches.
Kick-off at the Walkers Stadium is at 6pm.