The nature of Mir's injury has not yet been revealed.
MMAJunkie.com reported the news earlier Wednesday.
Mir and Cormier were scheduled to meet on a Strikeforce-promoted card in Oklahoma City. It was to be Strikeforce's final heavyweight fight.
Strikeforce has not yet determined whether another opponent will be selected to face Cormier, who was slated to compete in UFC after facing Mir.
Mir was expected to become the first fighter on UFC's roster to compete in a Strikeforce event. UFC and Strikeforce are owned by Zuffa LLC.
During his most recent fight -- on May 26 -- Mir was knocked out in the second round by UFC heavyweight champion Junior dos Santos.
Mir has a professional record of 16-6.
Cormier (10-0) won the Strikeforce Heavyweight Grand Prix Tournament on May 19 with a lopsided unanimous decision over former UFC heavyweight champion Josh Barnett.