Sidelined UFC lightweight champion Anthony Pettis will undergo surgery to repair his injured left knee, according to a report from Ariel Helwani on Wednesday's UFC Tonight.

The decision comes on the advice of specialists following extensive deliberation since Pettis withdrew from a planned December 14 title defense against former Strikeforce champion Josh Thomson earlier this month.

Pettis, 26, suffered a torn posterior cruciate ligament (PCL) in his left knee midway through his belt-winning performance against Benson Henderson at UFC 164. The injury is unrelated to the left knee injury that kept Pettis out of a featherweight title bout against Jose Aldo in August.

Pettis is slated to undergo surgery on Dec. 12 in Los Angeles, CA. A timetable for his return is unknown at this time, though Pettis guarantees to be back in action by summer 2014. Depending on the severity of the injury, PCL reconstructions generally require upwards of a six to nine month recovery period.

Due to extensive and various injuries, Pettis (17-2) has competed professionally just five times in the three years since he defeated Benson Henderson at WEC 53 to seize the WEC lightweight strap.