According to the Wisconsin State Journal, Bielema said during a taping of a show at WISC-TV studios in Madison on Thursday that Ball won't be ready when practice begins Monday. The Badgers open the season at home Sept. 1 against Northern Iowa.
Bielema gave no indication Ball would suffer any long-term problems. He said Ball's absence will spare last year's Heisman Trophy finalist hits and give other backs more repetitions.
Police said five men knocked Ball down as he was walking near campus early Wednesday. The men kicked him in the head and chest before fleeing.
Madison police on Wednesday said the attack was "unprovoked" and have not changed their evaluation, according to reports Friday in the Milwaukee Journal Sentinel.
"In terms of any precipitating event," Madison police department spokesman Howard Payne told the paper, "we are still investigating that and any other pieces of information that are out there that might lend some credence to why this may have occurred.
"We're hopeful the interviews we are conducting will (generate) more information as to whether that is a reality or not.
"With cases like this a lot of people speculate. We are trying to steer clear of that unless there is some reason to believe that speculation has some validity.
"We're trying to go with what we do know."
Players are scheduled to report Sunday, ahead of Monday's first scheduled practice.
"My concern right now is for Montee's health and well-being," Bielema said Wednesday in a statement. "Montee has been released from the hospital and is under the care and supervision of our sports medicine staff. We will continue to evaluate him as we approach the start of fall camp this weekend. I do expect Montee to make a full recovery."
Information from The Associated Press was used in this report.