Wales legend Scott Gibbs fears the current crop of international stars are being over-worked.
The 37-year-old, who retired four years ago having excelled in both codes, believes the current calendar of domestic and international features will ultimately reduce the quality of play.
He told the Wales on Sunday: "I look at the intensity of the rugby calendar and I do wonder how the modern player just keeps doing it.
"You only have to look at how poor the quality of some of the play is to see that the players cannot play to the level their talent deserves.
"I look at Shane Williams, somebody who has been absolutely outstanding this year, and I worry and wonder how is he going to keep performing at that level. The answer is: he won't. Why? Because you just can't. Professional players have to rest."
England flanker James Haskell insists he will be pushing as hard as possible to be back from a toe injury in time to make his case for a spot in Eddie Jones' side looking to defend their RBS 6 Nations title next year.