New Welsh selection policy, in a bid to keep the best talent playing in the country, has stated any player taking up a new deal with a team outside Wales must have won 60 caps to be able to play for the national side.
The move comes after half-back Webb confirmed he would be making the switch to French outfit Toulon at the end of the current campaign.
With Webb, who has 28 caps to his name, now in a position to miss out on the next World Cup, former British & Irish Lions star Williams has admitted it is a concern.
He was quoted by the BBC, saying: "It would be very difficult thinking Rhys Webb isn't going to be there [the World Cup] and other players along the line as well.
"We've got two years before the World Cup and I'm sure there will be opportunities for other experienced players to go elsewhere."