It is unclear when Butler suffered the injury. He played 37 minutes in Wednesday night's loss to the Orlando Magic.
Butler's injury is the latest setback to a Bulls team that has been battered by injuries all season. Wednesday's game marked just the fourth time the Bulls had a completely healthy roster and just the 20th game that all five starters, Derrick Rose, Joakim Noah, Pau Gasol, Mike Dunleavy and Butler had played together.
Butler's injury comes at a bad time for the Bulls. They entered Thursday's game in fourth place in the Eastern Conference, just a game ahead of the Washington Wizards. Butler's injury also hinders Tom Thibodeau's hope that the Bulls could find some consistency with Rose coming back for the first time in almost six weeks from a meniscectomy on Feb. 27.
Rose was 5-for-15 with 12 points and four rebounds in 20 minutes of play. Thibodeau said before Thursday's game that Rose would play about that much.
Rose scored nine points in 19 minutes against the Magic.