MLS is a 'single entity' league with the CBA governing player contracts, salaries and legal status. Contracts are owned by the league rather than individual clubs.
The players union, the MLSPU, have been demanding major changes in the structure of the league, which it calls a cartel that restricts freedom of movement, while MLS insists it has the right model to take the sport forward in North America.
Former U.S goalkeeper Kasey Keller and current national team striker Landon Donovan on Tuesday both made critical comments of MLS on the website of the international players union, FIFPro.
"What we are looking for are the same basic rights that players enjoy in other leagues around the world," said Keller, who has played in England, Germany and Spain.
"We have made great strides in developing the game in the United States. But we can't truly compete internationally, either for players or fans, with a system that is so radically different than other leagues around the world."
The union argues that MLS's structure and contracts breach FIFA regulations - and the FIFPro statement made a number of charges against the league.
Failure to reach a new CBA by the end of the month raises the prospect of an employers' lockout or a players' strike.
Donovan, who is currently on loan from L.A Galaxy to English club Everton, said a lockout would be particularly damaging in a year with the World Cup finals in South Africa.
"The league shutting down MLS in February would do real damage to the development of the game in the United States and to our efforts to prepare for South Africa," he told the FIFPro site.
"It is difficult to understand why the owners would take this course, when all we are asking for are the same rights enjoyed by other players around the world, not just in the biggest leagues, but in leagues of all sizes."
MLS President Mark Abbott rejected the charges.
"The statement regarding MLS issued by the MLS players today contains many inaccuracies including the false assertion that MLS is not compliant with the FIFA regulations," he said.
"MLS is in fact operating in compliance and the players are simply wrong on this point. Also, contrary to the union's claims, it has been proven in federal court that the MLS business structure is legal and does not operate as a cartel.
"Moreover, any discussion about a lockout, players strike or other work stoppage is premature and frankly counterproductive to our ongoing mutual commitment to reach an agreement between management and the players."
