The experienced pair failed to recover from respective hip and ankle injuries and were replaced by Edinburgh duo Allan Jacobsen and Allister Hogg.
"Williams, however, is still hoping his side can notch their first win of the tournament."
He said: "I'm very disappointed that two of our most experienced players are out and there's not doubt it's a huge blow for Scotland.
"But it's fantastic to have two fine players coming in their place. Ally Hogg has had a bit of experience and Allan Jacobsen deserves his start because he's been playing so well for Edinburgh.
"So it's a case of one door closing and another one opening.
"Tom is a world class player there's no doubt about that but we just have to get on with it.
"Putting pressure on all the players to perform but the pressure is also coming from within the team, not just from the outside."