Brown will be sidelined for six weeks with scans revealing the fracture.
The injury is unrelated to those that Brown sustained in 2011.
He will have surgery on Wednesday night.
The recovery timeline means Brown will also miss the opening two rounds of the premiership season.
Brown was injured in a collision with teammates during a training drill.
He remained conscious during and after the collision and walked off the field without assistance.
Lions General Manager - Football Operations, Dean Warren, said: “It’s obviously disappointing for Jonathan.
“He’s had a very good pre-season and was in excellent shape heading into 2012.
“Fortunately the fracture is unrelated to the injuries he sustained last year.
“He’ll have surgery tonight and our focus as a Club is clearly on his health and wellbeing.
“He’ll be determined to return as soon as possible but we’ll ensure he receives the appropriate clearances when the time is right.”
