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's preparations for the Autumn International series were potentially dealt a further blow as Maro Itoje was forced to undergo an x-ray after Saracens' European victory over Scarlets on Saturday night.