Details of the altercation between A’s outfielder Danny Valencia and designated hitter Billy Butler emerged Monday, with Valencia facing possible discipline later in the day for his role in the incident. The physical confrontation between the two teammates left Butler unable to play in the weekend series at Chicago because of a possible head injury.

Two players who witnessed the event and helped break up the fight spoke to The Chronicle on the condition of anonymity because they didn’t want to appear to be taking sides. One said that Valencia and Butler both played roles in the incident: Valencia should not have punched Butler. And Butler should not have provoked Valencia by potentially costing him an endorsement contract.

According to the players, the incident occurred before batting practice Friday when an equipment representative quizzed Valencia about a pair of off-brand spikes in Valencia’s locker. Valencia previously had been told not to wear those shoes in games.

Valencia told the representative that he only uses the non-issue spikes during pregame workouts. According to multiple sources, Butler, who has an equipment endorsement with a different company, jumped in to tell the equipment rep that Valencia was lying and regularly uses the non-standard spikes. Butler allegedly told the representative that the company should drop Valencia’s endorsement deal.

Endorsement deals are typically worth between $10,000 and $20,000, sometimes more.

After the rep departed, the players said, Valencia confronted Butler and told him, “Don’t you ever loud-talk me in front of a rep. That was wrong,” and walked aggressively toward Butler. Butler turned around, took a couple steps toward Valencia, and according to both witnesses, said, “I can say whatever I want and your bitch ass isn’t going to do anything about it.”

One player said that the men leaned in, bumped heads and then started pushing each other, then Valencia started swinging and hit Butler in the temple. After the players broke things up, Butler told the players he was OK.

Butler, who then took part in batting practice, was not scheduled to play that night against right-hander James Shields, but he missed the next two games against left-handed starters with what manager Bob Melvin described as nausea and vomiting.

Butler has not responded to repeated texts from The Chronicle, and Valencia declined to comment when asked about the incident after Sunday’s game in Chicago.

It’s unclear what discipline Valencia might face, although it is possible he could be suspended or even released. Teammates Coco Crisp and Yonder Alonso sent supportive tweets his way, as did former Royals teammate Eric Hosmer, who also played with Butler for a number of years.

Clubhouse incidents are not uncommon, although players seldom are injured as a result. And Butler and Valencia have a long-standing loud, joking, back-and-forth relationship that dates back to their time as teammates in Kansas City.

“Those two go at it every day, so in that way, it was pretty normal,” one player said. “If Bill had dropped it, Danny never would have done anything. But he kind of kept egging him on. I think it’s fair to say he was verbally provoked, very much so. I’m worried about what might happen to Danny now, but at the same time, you don’t want to see anyone get hurt.”

Susan Slusser is a San Francisco Chronicle staff writer. Email: sslusser@sfchronicle.com Twitter: @susanslusser