Chicago Bulls players contacted the National Basketball Players Association on Sunday because of what they felt were extreme tactics by new head coach Jim Boylen, league sources told Yahoo Sports, the culmination of a growing divide between the coach and his players.

After a close win over the Oklahoma City Thunder on Friday and a 56-point rout Saturday night by the Boston Celtics, Boylen, finishing his first week as head coach, called for a Sunday practice — an NBA protocol no-no with the Bulls just completing a back-to-back.

After taking the helm from fired coach Fred Hoiberg on Monday, Boylen held three two-and-a-half-hour practices in his first week that included extra wind sprints and players doing military-style pushups. Calling for another lengthy practice after the back-to-back led to a near-mutiny and caused the players to reach out to the union, sources said.

Boylen had already gotten under his players’ skin by issuing the ultimate indignity: subbing out all five players on the floor twice in Saturday’s loss. In the first half, the Bulls fell behind 17-0, so the full lineup change was understandable.

In the second half, the Bulls had only been outscored 5-3 when Boylen subbed everyone out and the players felt embarrassed.

Calling for a Sunday practice was apparently the last straw.

Late Saturday evening one of the team’s veterans, sources said, initiated a group text relaying that if any player showed up to the facility on Sunday prepared to practice, he would fine them personally, and the players agreed.

A player contacted Boylen to alert him of the team’s decision, but the coach refused to relent, sources said.

Bulls head coach Jim Boylen questions a call during his debut Tuesday night against the Pacers. (AP) More

That’s when the players compromised among themselves, agreeing to show up but with no intention of practicing, sources said. Robin Lopez was the leader of getting the players to back down, sources said, although he wasn’t in the initial group text.

Boylen has had numerous verbal confrontations with players since the first week of the season, sources said, with executive vice president John Paxson and general manager Gar Forman being present for many of them. Boylen was Hoiberg’s “bad cop” and clearly embraced the role in his three-plus seasons as associate head coach.

Hoiberg gave Boylen the latitude to run film sessions and practices, and players have privately stated it was Boylen coaching the team, not Hoiberg.

Following Boylen’s debut Tuesday in a losing effort to the Indiana Pacers, he went against the usual postgame protocol by forcing the team to watch film while their emotions were still running high. It is customary to watch film the next day after everyone, including coaches, has decompressed and is ready to be critiqued.

Players felt like they were being treated like high school athletes and those feelings of disrespect escalated when Boylen told the media the players needed to get in better shape, sources said.

Boylen clearly wanted to put his stamp on the Bulls after taking over. And even though those close to Boylen say he never intended on running the players through a rigorous practice on Sunday, several players felt his act of pulling the starters for the final 21 minutes of the Boston game was a sign he was going to work them hard in practice.

When Boylen arrived Sunday, the players stood and told Boylen they weren’t practicing, sources said, with the sides meeting to express their issues. Zach LaVine and Justin Holiday were the most vocal, sources said.

Boylen repeatedly referenced his days on the San Antonio Spurs staff and instances in which coach Gregg Popovich pulled all five players off the floor to send a message, sources said.

A player responded, sources said, telling Boylen in essence that they aren’t the Spurs and, more importantly, he isn’t Popovich.

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