A new global calendar is being discussed for the start of the 2020 season, with England's top-flight schedule set to change.
New plans suggest the Aviva Premiership could run from September to June, with international tours then being staged in July.
That would leave the players just the month of August to recover and Roberts believes they would struggle to cope with the demands.
He said: "The Rugby Players' Association stance is pretty clear - rest is equally as important as playing.
"The lads will tell you that when you have a good off-season and rest and get a good pre-season in, that's when your body feels at its best.
"If you are going to extend the season you have to remember that international players will be playing into July. You can't expect them to have four weeks off and maybe three weeks pre-season. It is crazy.
"In-season breaks aren't the same because you will still be in training. That mental time away from the game is equally as important."