Manchester United's schedule of four matches in a week is "not physically possible" the boss of world players' union FIFPRO has warned.
The rescheduling of the Red Devils' Premier League match against Liverpool after fan protests at Old Trafford last Sunday means they will now play on May 6, 9, 11 and 13.
A FIFPRO report published on Thursday found top players in Europe are playing up to 80 per cent of their minutes in the so-called 'critical zone' where they are more susceptible to injury.
Congestion caused by the coronavirus pandemic has meant regulars at club and international level such as United captain Harry Maguire rack up the large majority of their playing time without the recommended five days' rest between appearances.
Cumulative exposure to matches in this 'critical zone' – combined with international travel and shortened off-season and in-season breaks – constitutes an issue for a player's health and career longevity, FIFPRO said.
The report looked at 265 players in 43 domestic leagues, and drew on data from FIFPRO's Player Workload Monitoring (PWM) platform.
It found that 73 per cent of the minutes Maguire played between the start of May last year and the end of January this year were within the critical zone.
Maguire's club are now embarking on a hugely congested run-in to the end of the season.
FIFPRO general secretary Jonas Baer-Hoffmann has often spoken about how the conflicting interests of competition organisers conspire to place an unacceptable burden on players, and said on Thursday: "There's a beautiful example with Manchester United coming up now.
"They play Thursday, Sunday, Tuesday, Thursday. Four games, May 6 to the 13, including travel. There were specific circumstances why this one game had to be delayed. But, I mean this is just not physically possible."
Manchester City's Ilkay Gundogan criticised the new 36-team format UEFA has approved for the Champions League from 2024 onwards, describing the minimum of four extra games in the group phase as "the lesser of two evils" compared to the breakaway European Super League, which was launched on April 18 but quickly fell apart.
FIFA is also trying to find a new slot in the calendar for its expanded Club World Cup competition. The first edition was due to be played in China this summer but was indefinitely postponed in order for the rescheduled Euro 2020 and Copa America tournaments to take place.
Baer-Hoffmann has warned that without immediate action there will be "hundreds of injuries".
"We can't just accept waiting until the review of the (post-2024) calendar," he said.
"All these competition interests will collide and health will again be sucked into the second or third priority.
"There hasn't been a conversation where people genuinely look at the calendar and all the competitions in their entirety, and they say 'OK let's for one hour forget about my competition. Let's look at that entire thing and see what works'. That never happens."
FIFPRO is seeking guarantees around the length of off-season and in-season breaks, gaps between games and pre-season conditioning periods.
As well as the example of Maguire, its new report found three-quarters of the players in the study operating in 'winter leagues' which traditionally run from August to May, did not get the five-week off-season break which FIFPRO considers to be the minimum required.
United midfielder Bruno Fernandes had just 20 days off between his club's last Europa League match on August 16 and representing Portugal in the Nations League on September 5. It also found that 69 per cent of his minutes were played in the critical zone.
Bayern Munich defender Niklas Sule and Paris St Germain forward Kylian Mbappe had just 11 and 13 days' break respectively.
