Bulls coach Fred Hoiberg said the injury occurred late in Monday night's loss to the Utah Jazz when Butler went knee to knee with a Jazz player. He dressed out with the rest of his teammates and sat on the bench Wednesday night.
"I don't know," Butler said, when asked if the injury could linger. "See how it feels [Thursday]. I'm going to wake up and get some treatment and try to get back out there."
Hoiberg said he is hopeful that Butler's injury isn't long term and sounded confident that Butler will play Friday against the Denver Nuggets. No MRI has been scheduled.
Wednesday's contest marks the first missed game of the season for Butler. He had been open about the fact that he wanted to play in all 82 regular-season games after making a bet with his trainer before the season.
"It's disappointing," Butler said. "But I got to think big picture. I got to take care of my body. My trainer was here, and he was like, 'Look, you got to do what's right for your body,' so I talked to the coaches. Everybody thought it was the best idea if I sat this one out."
Butler did not want to reveal what the winner of the bet received.
In other injury-related news, veteran Mike Dunleavy rejoined the Bulls before the game after a two-day practice stint with the Santa Cruz Warriors, the D-League affiliate of the Golden State Warriors.
Dunleavy has yet to play in 2015-16 after offseason back surgery but is hopeful to make his debut at some point this weekend.
"It was a productive trip," Dunleavy said. "Got what I needed to get out of it."
Dunleavy was unclear on when he will return to the floor because he still has to work out how many minutes he will be able to play once he does return.
"I'm pretty close," Dunleavy said. "Just a matter of seeing how I'm feeling the next couple days and probably start having conversations with the medical staff in terms of strategy and minutes and how we're going to structure this thing. Obviously, I'm really close and hope to be back soon."