In Eastern Europe, the problem is more pervasive. Results of a 2012 player survey conducted by FIFPro, the international players’ union, found that in 12 Eastern European countries, roughly one in six players reported having been forced to train alone, while about 40 percent of players in Poland specifically said they knew of at least one such incident.

Mr. Stefanovic, the Slovene player representative, said the survey offered only “a hint” of how common such intimidation tactics were. He cited the case of the Serbian midfielder Andrija Zivkovic, one of the continent’s most promising young players, who starred at the FIFA U nder-20 World Cup last year and was a centerpiece of the decorated team Partizan Belgrade.

Earlier this year, Mr. Zivkovic, 19, refused to sign a contract extension with Partizan after drawing interest from richer clubs abroad, Mr. Stefanovic said. He was summarily sent home from the club’s training camp and has since been barred from practicing with the first team.

“He’s literally running circles by himself,” Mr. Stefanovic said. “If this happens to one of the best players, imagine what is happening to the other poor guys.”

Much of the problem, Mr. Stefanovic said, is a lack of standard professional contracts for players. Mr. Stefanovic said that in Romania, for example, players are typically assets of the club, which means that even if a club goes bankrupt, players are not allowed to find new clubs because all assets are frozen.

Mr. Stefanovic added that as far back as 2011, FIFPro, which represents players’ interests but lacks collective bargaining power or leverage with individual leagues, had asked European soccer’s governing body, UEFA, to consider making standard contracts a requirement. The union has seen no progress, he said, adding, “The problem is getting worse, not better.”