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MESA, Ariz. — Kris Bryant, Javier Baez and Addison Russell will start the season in the minor leagues.

Because of one of those moves, the Cubs face a battle with the Major League Baseball Players Association.

The players union knocked Monday’s decision to demote Bryant, the top prospect who leads the majors with nine home runs and is batting .425 in spring training. By keeping Bryant off the 25-man roster for at least 12 days starting Sunday, the Cubs would retain Bryant’s rights through at least 2021.

“Today is a bad day for baseball,” the MLBPA said in a statement. “I think we all know that even if Kris Bryant were a combination of the greatest players to play our game, and perhaps he will be before it’s all said and done, the Cubs still would have made the decision they made today. This decision, and other similar decisions made by clubs, will be addressed in litigation, bargaining or both.”

Before the statement was issued, Cubs President Theo Epstein continued to stress the 6-foot-5 Bryant, 23, will work on his defense at third base at Triple-A Iowa in addition to playing the outfield.

“I thought he played very well at third after he came back from missing a bit of time with the arm issue,” Epstein said. “I thought he moved very well. I think he’s on his way to being a very good third baseman and will continue to work on it.”

Epstein added there were no hard feelings toward agent Scott Boras, who two weeks ago questioned the Cubs’ commitment to winning, saying Bryant should have been promoted in September after hitting 43 home runs in the minors.

“Scott was doing his job for his player,” Epstein said. “I control only what I can control, which is how the organization handles things, what we say publicly.”

There was a tug of war over where Baez, the slugging second baseman, should start the season. It pitted those who want him to cut down his high strikeout rate against those who believe he could benefit more by working with new Cubs batting coach John Mallee while providing top-notch defense at second.

“It was a healthy debate,” said Maddon, a staunch backer of Baez, who hit 20 home runs at Iowa but struck out 95 times in 213 at-bats during a two-month stint with the Cubs. “It just comes down to what is the right thing to do for the player and the team.”

Neither Baez nor Bryant was available after the cuts were announced, but pitcher Kyle Hendricks said they took the demotion in stride.

“They understand what this game is,” Hendricks said. “They’re going to work that much harder. And when they get back, they’re going to be that much more hungry.”

Despite demoting Bryant, Baez and Russell, Epstein didn’t discount their abilities.

“I probably can be in this game for a long time and not send down three players that talented on the same day ever again,” Epstein said.

With Baez out of the mix, Tommy La Stella and Arismendy Alcantara likely will share second base, and the Cubs feel La Stella’s defense at third is adequate to spell Mike Olt on occasion.

With outfielder Chris Denorfia probably headed to the disabled list, that leaves four players — outfielders Matt Szczur, Junior Lake and Ryan Sweeney, along with non-roster infielder Jonathan Herrera — competing for two spots.

Lake has impressed Maddon nearly as much as Alcantara, whom he called “a beautiful man” for his ability to play six positions.

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