http://fiveouncesofpain.com/2009/03/06/injury-to-frank-mir-forces-cancellation-of-ufc-98-rematch-vs-brock-lesnar/

A planned rematch between Brock Lesnar and Frank Mir for UFC 98 on May 23 that would have unified Lesnar’s UFC heavyweight title and Mir’s interim UFC heavyweight has been canceled.

Multiple sources have confirmed to FiveOuncesOfPain.com that the UFC was recently informed by Mir’s camp that he was forced to withdraw due to injury. Five Ounces of Pain originally had been informed that Mir had suffered a back injury but MMAWeekly.com has since reported that Mir actually suffered a knee injury and has already undergone surgery.
“Its just like this shoe but the exact opposite, not like a evil version or anything just for the other foot… ”