Roethlisberger didn't recall how it happened, but said tests on Monday revealed the break. He also broke the same thumb in 2005.
The Steelers have a bye this week after their 24-17 win over the Bengals and will return to action on November 27 in prime time against Kansas City.
"Luckily we have the bye week," Roethlisberger said. "I don't have to throw right now. It will be a good time for it to rest. The big thing is just getting used to throwing with a splint on it again and a glove, which I have done before."
Roethlisberger, who has played through numerous injuries during his career, has completed 63.3 percent of his passes for 2,877 yards with 16 touchdowns and nine interceptions for the AFC North leaders.