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.

According to MMA Weekly’s report, the surgery will sideline Mir for the next four-to-six weeks.

There is no word as to whether Lesnar will remain on the card against a new opponent or if he will be taken off the card entirely.

The UFC had just officially announced Lesnar vs. Mir II via its website earlier this week. The much-anticipated rematch had been anticipated to draw a large buyrate on pay-per-view.

The first meeting at between Lesnar and Mir took place at UFC 81 in February of 2008 and ended at 1:30 of round 1 after Lesnar was caught in a kneebar by Mir. Competing in just his second pro fight, the former NCAA heavyweight champion was forced to tap despite the fact that he had knocked Mir down and was reigning down punches on him just prior to getting trapped in the submission hold.

The two had been slated to fight each again other after both secured wins in the first round of a heavyweight title unification tournament. However, a Lesnar vs. Mir rematch in the finals of the tournament had been considered an improbability going in.

But the odds of a second fight between the two changed greatly after Lesnar won the heavyweight title from defending champion Randy Couture this past November at UFC 91. Couture, fighting for the first time after a 15 month layoff was TKO’d by the former WWE superstar at 3:07 of round 2.

The rematch between Lesnar and Mir became a reality following Mir’s upset victory over defending interim heavyweight champion Antonio Rodrigo Nogueira at UFC 92 this past December. Displaying improved standup, Mir was able to TKO the former PRIDE heavyweight champion at 1:54 of round 2.