There is no talk of imminent strike action, though one RPA source confirmed "nothing is off the table".
However, English-based players, already facing one of the toughest schedules in the world, are bitterly frustrated by Premier Rugby's failure to consult them, before announcing plans.
One well-placed source said: "It's a disaster. There is no clarity on what form the rest periods will take and even if a player is given a mid-season break, they will never be completely 'off'.
"Can you imagine strength and conditioning coaches waving goodbye to a player in mid-season and allowing them to put their feet up for two weeks. It's never going to happen.
"The consequences of asking players to effectively be on duty for 11 months a year will almost certainly increase the likelihood of mental illness, as well as physical problems. At the point when rugby should be looking to extend rest periods, it is looking to reduce them. Yet again, player welfare has been the big loser."
Senior representatives from the RPA - including chief executive Damian Hopley, who is also chairman of the International Rugby Union Players' Association (IRUPA), who agreed the international window change at a World Rugby meeting in San Francisco in January - will seek guarantees over release to prevent burn-out.
With international players facing the additional burden of a summer tour, Lions trip or World Cup cycle, England stars face the possibility of an eye-watering 11-month season as standard.
RPA chairman and veteran Northampton lock Christian Day, speaking in a personal capacity and not on behalf of the RPA, is frustrated by the lack of consultation.
"A 10-month domestic season is frightening enough for me," he said. "But with the prospect of an international season becoming 11 months with a summer tour, it begins to stretch the boundaries of what's possible.
"If you add in the five-week rest period, which the RPA fought really hard for as a mental and physical recharge for the players, it basically means there is no off-season. It is an incredible ask of our elite players.
"At the moment, we just don't have enough information. It's all well and good saying 'Don't worry, there will be welfare', but we need cast-iron assurances of when players are going to be rested. Even then, you are reliant on coaches.
"Up until now, a strike hasn't happened and let's hope it never has to. It's happened in American football, but it's not a particularly British thing to do.
"Up to now, Premier Rugby, the RFU and ourselves have worked well together. We've been reasonable, not made excessive demands and tried to grow the game for the good of the game.
"As long as that carries on, I don't think a strike will happen, but we need that to carry on and have that discussion around the table. We can't just have one element doing what it wants.
"I suspect Premier Rugby haven't planned it all yet. They could easily have shifted to an October start and June finish.
"That isn't something they are currently choosing to look at. We need to understand why."
The move to extend the English domestic season, while condensing the Six Nations into six weekends from seven, has led to concern among players, coaches and backroom staff.
One Premiership coach said this week: "It is an unworkable schedule, with players unable to get enough rest. The only way to get an extended rest period now is to pick up a long-term injury."
Scotland, whose players suffered eight concussions in their penultimate two Six Nations games, intend to block the move to give the Six Nations only one fallow weekend.
SRU chief executive Mark Dodson said: "Reducing the Six Nations from seven weeks to six is a retrograde step and dangerous for player welfare. We will look after our boys."
Premiership Rugby chief executive Mark McCafferty, however, insists the new schedule would actually improve player welfare, with a revamped cup competition during the Six Nations designed to enable clubs to rest and rotate squads.
McCafferty said: "It's worth noting the IRUPA were part of these discussions and had their say. At a domestic level, those discussions are developing now about the detail and given the detail is only coming into effect in two or three years, I suspect those discussions will continue for some time and the mood might change.
"There is no reason for clubs not to look after players. For 95 per cent of our players who are not involved in international rugby, this will help enormously.
"If we are removing Premiership fixtures from the Six Nations period in February and March, and expanding the development cup during that period, it gives even more opportunity for clubs to rest or rotate those players, depending on their individual needs.
"That's going to be a real benefit to players and clubs, and we need to go through that detail with them. Mid-season is an ideal time to take a breather, before you get into the business end."
McCafferty points to the fact there will be no increase in the 32-game per season limit for players, while insisting the five-week mandatory break will stay "in the short and medium term", but it may be hard to convince players of the merits of extending the season by a month.
Saracens director of rugby Mark McCall admits it is a tough balancing act between promoting the club game and player welfare.
"You've got three competing interests - player welfare at the front of it, then international rugby and club rugby. We've got to be careful that international players don't get burnt out by being involved all the time."
