The rapid evolution of fantasy sports from casual season-long recreation to high-stakes daily Internet games has given U.S. professional sports another source of revenue as well as a headache in determining player-participation policies.

Over the last year, all four major sports leagues signed sponsorship deals with daily fantasy websites through players, teams, or the league itself. They are now promoting a business that, while legal in most of the United States, resembles sports gambling because participants profit by correctly predicting success.

As the Major League Baseball season gets underway Sunday, players for the first time will not be permitted to join paid fantasy baseball leagues, following an agreement between the sport and its players' union. The NBA and its players' union disagree as to whether players are prohibited from participating.

"The leagues are fully embracing and monetizing daily fantasy, but simultaneously are putting rules in place to bar league athletes, coaches, and trainers from participating," said Ryan Rodenberg, who teaches sports law at Florida State University. Sports leagues are facing a "policy tightrope," he said.

Fantasy games are legal in most states because of a 2006 federal law declaring them skill-based. Entry fees for one-day games were $245,000 in 2013 and are expected to rise to $11 billion by 2018, according to Eilers Research L.L.C.

The Wall Street Journal reported Friday that the Walt Disney, Co., which owns ESPN, has invested $250 million in DraftKings Inc., which announced a deal with MLB on Thursday. A unit of Philadelphia-based Comcast Corp. invests in FanDuel Inc., a competitor to DraftKings. The 76ers have a marketing agreement with DraftKings.

The most recent collective bargaining agreements between leagues and players - signed before daily fantasy's growth - ban gambling within their own sports. Fantasy, however, is more difficult to classify.

The pastime, where team "owners" draft players and compare their statistics to determine a winner, has been growing since the 1980s as a season-long venture. In daily fantasy, players draft new teams every day, winning as much as $2 million.

The leagues' stands create a question of, if daily fantasy poses no threat to the integrity of sports, "then why the ban," asked Rodenberg.

Eagles running back DeMarco Murray, whose fantasy statistics with the Dallas Cowboys were among the NFL's best last season, said during Super Bowl week in Arizona that he does not play fantasy football, but some of his teammates do.

"I don't want to name them, but they talk about it quite often," Murray said during a news conference panel discussion.

"They're just having fun, just like everyone else," Murray said later. "I don't think they would throw a game or try to get cute."

Former NBA player Roger Mason Jr., now deputy executive director of player relations for the National Basketball Players Association, said that to his knowledge, players are not playing daily fantasy basketball, and that it remained under the radar of most.

"It's gambling," Mason said during All-Star Weekend in New York. "If you're putting money down and winning money in return, that's gambling."