The injury bug has once delayed a heavily-anticipated UFC title bout.

UFC lightweight champion Anthony Pettis has withdrawn from his upcoming title defense against former Strikeforce titleholder Josh Thomson at UFC on FOX 9 next month due to a knee injury, the FOX Sports Live Twitter account posted Sunday night.

The December 14 event will now instead be headlined by a flyweight title rematch between Demetrious Johnson and perennial title contender Joseph Benavidez.

Johnson and Benavidez battled to crown the UFC’s first flyweight champ at UFC 152 last September, with “Mighty Mouse” squeaking out a hard-fought split decision.

The 125-pound title tilt was originally supposed to headline The Ultimate Fighter Season 18 Finale, which is scheduled for November 30.

That card, the TUF 18 Finale, will now be headlined by two former 155-pound title challengers in Nate Diaz vs. Gray Maynard.

While this is technically the second bout between the two established lightweight competitors, with Maynard winning a split decision back in back in January 2010, they two fought in an exhibition bout as part of the TUF season 5 tournament back in 2007.

Although the result is not reflected on either fighter’s professional record, Diaz won that contest when he locked up a guillotine choke in the second round.

Coincidentally, Thomson earned his title shot against Pettis when he became the first fighter to knockout Diaz at UFC on FOX 7 in April.

After getting out-wrestled in his UFC debut against Clay Guida in July 2011, Pettis has rattled off four straight wins, culminating in a title win over Benson Henderson back at UFC 164 in August. 

From a fan's perpsective, are Johnson vs. Benvidez II and Nate Diaz vs. Gray Maynard III worthy headlining fights for free fight cards?