The bout will serve as the co-main event of the evening, sources have confirmed.
Torres (37-3) held the WEC's 135-pound belt from Feb. 2008 until Aug. 2009, when he lost by KO at the hands of Brian Bowles. That was his first loss in nearly six years.
Surprisingly, he then suffered a second straight setback when he tapped out due to a guillotine submission against Joseph Benavidez in March. Despite the back-to-back defeats, however, Torres is still considered one of the best bantamweights in the world.
In Valencia, he faces a quality veteran who is in the midst of a three-fight win streak, defeating Seth Dikun, Coty Wheeler and most recently, Akitoshi Tamura. Valencia is 12-5 all-time and 5-3 in his WEC career.
WEC 51 takes place on Sept. 30 from 1st Bank Center in Broomfield, Colorado. The event is likely to be headlined by a featherweight title defense by champion Jose Aldo, though an opponent has yet to be decided upon.