The Bulls said Rondo underwent a subsequent exam and X-rays confirmed the thumb fracture. His injury will not require surgery.
The eight-seeded Bulls beat the Celtics 111-97 on Tuesday to take a 2-0 lead in their best-of-seven series against the No. 1 seed in the Eastern Conference. Rondo just missed with a triple-double, finishing with 11 points, 14 assists and nine rebounds, and added five steals.
Rondo is averaging 11.5 points, 10 assists and 8.5 rebounds in the Bulls' two victories over the Celtics.
The series resumes Friday night in Chicago.