Jackson, who played in Cleveland's first four exhibition games, first told the team on Tuesday the wrist was bothering him. He played 11 minutes in Thursday night's game against San Antonio, and an examination on Friday revealed the fracture. Jackson said the wrist will be in a cast for about 10 days.
"I really don't know [how the injury occurred]," Jackson said. "Every time I taped it up, it felt fine. I can take a lot of pain. I finally asked them if they could check it out."
Jackson, who helped Kansas to the national championship last season, was selected in the second round by Miami. The Cavaliers acquired him for a 2009 second-round pick.
The 6-foot-8, 250-pound Jackson was averaging 3.5 points in the exhibition season. He was 7-of-11 from the field and had nine rebounds.
"He's had a good camp," Cleveland coach Mike Brown said. "Now, he has to figure out the other side of it, to stay in shape and keep learning."
Jackson, 23, averaged 11.2 points and 6.7 rebounds in 35 games for Kansas last season.