Bruins General Manager Peter Chiarelli confirmed Kelly's right fibula was broken on a play during Saturday's 3-2 home victory over the Penguins in which Pittsburgh's Pascal Dupuis delivered a two-handed slash to the Boston player while along the boards.
Canada's TSN and the Boston Globe had reported earlier Kelly sustained a broken ankle.
Kelly has posted three goals and seven points in 28 games for the Bruins this season. It was his his second recent leg injury -- in March broke his left tibia, keeping him out of the Bruins' lineup until late in the post-season.
Orpik later on was carried off the ice on a stretcher after being punched in the face by the Bruins' Shawn Thornton, who now the subject of an NHL disciplinary hearing.