The Eagles made clear they wanted Tanaka to stay with the team, not just because they would lose their top pitcher so soon after winning their first title but because they would receive far less for him than hoped under the new posting system. The Eagles were hoping to generate at least $50 million in compensation, roughly what Matsuzaka’s and Darvish’s teams received.

Any team willing to pay the release fee can negotiate with Tanaka during a 30-day window. The team that signs Tanaka will pay the Eagles the fee in installments. There is no penalty for making a bid and then failing to sign the player.

In letting him leave, Rakuten may have weighed the odds that Tanaka will continue his mastery over batters in 2014 and beyond. It is unlikely he will ever have as dominant a season as he did in 2013; for one thing, he has thrown many innings the past few years. For instance, in the sixth game of the Japan Series this year, he threw 160 pitches and refused to come out of the game. In the clinching seventh game, he appeared in relief. That sequence alone may give some major league teams pause as they ponder how much they might be willing to offer Tanaka in a free-agent contract.

According to Sponichi, a Japanese sports publication, Rakuten may try to form a partnership with whatever team signs Tanaka. The Yomiuri Giants did that after their slugger Hideki Matsui signed with the Yankees before the 2003 season.

As dominant as Tanaka has been during his time with the Eagles, whom he joined after finishing high school, there is no guarantee he will fare as well in the United States. His fastball is unlikely to overpower major league batters, who routinely grapple with pitches nearing 100 miles per hour. Matsuzaka, who is 53-40 in the majors, and Darvish, who is 29-18 after two seasons in the United States, have had to adjust their pitching styles.

Matsuzaka, in particular, encountered adversity after his first two seasons in Boston.

Japanese pitchers also have to learn how to throw off mounds with balls that are different from those in Japan, and deal with umpires with different strike zones. Most Japanese pitchers also speak limited English, making it difficult to communicate with catchers and coaches.

Japanese baseball fans are likely to continue rooting for Tanaka in the United States because they take pride in seeing their stars play overseas. But his departure is likely to stoke fears that Japanese professional baseball, a source of national pride, is turning into a second-class league.

“They are becoming a feeder system for the M.L.B.,” said Robert Whiting, who has written several books on Japanese baseball. “Japanese pro baseball has all but disappeared from prime time network television.”