Prospective Lions skipper Thomas suffered a fractured thumb in Wales' dramatic 24-18 victory over France and is set to have an operation on Monday to insert a plate and screws to help mend the break.
Thomas, nicknamed Alfie by his team-mates, will have to watch from the sidelines as Wales chase their first championship title since 1994 and a first Grand Slam since 1978.
"You are not going to replace Alfie. We are lucky we have so many talented backs but as a man, he is our skipper, he is our leader," said Williams.
"It is going to be a big, big loss to us. Hopefully we can pull through that and whoever comes in will step into his shoes and come in.
"You can't speak highly enough of Alfie. He has emphasised what it means to be a squad, that no-one is bigger than anyone else."