By Keith Idec

LAS VEGAS – Joseph Diaz Jr. didn’t wow the crowd Saturday night, but the undefeated featherweight contender withstood an admirable challenge from Rafael Rivera.

Diaz overcame some early difficulty against a late replacement from Mexico and won a 12-round unanimous decision as part of the Canelo Alvarez-Gennady Golovkin undercard at T-Mobile Arena.

Each of the three judges – Tim Cheatham (119-109), Burt Clements (120-108) and Glenn Feldman (119-109) – scored the WBC elimination match for the 24-year-old Diaz (25-0, 13 KOs), a 2012 American Olympian from Downey, California.

Mexico’s Rivera (25-1-2, 16 KOs) gave Diaz a tougher time than the scorecards indicated, especially early in the fight, despite that he accepted it Tuesday as a late replacement for injured contender Jorge Lara.

Diaz connected with a left uppercut in the 10th round that turned Rivera’s head around. By that point, Diaz had taken control of the fight.

Diaz enjoyed one of his best stretches in the fight during the second half of the ninth round. He landed left hands and right hooks in combination and mostly prevented Rivera from coming forward.

Rivera landed an overhand right and a followed it up with a left hook that moved Diaz backward near the end of the sixth round. Rivera hammered Diaz with a straight right hand that snapped back his head just before the end of the fifth round.

Diaz drilled Rivera with a straight left hand around the 40-second mark of the third round. Rivera responded by landing two hard right hands to Diaz’s head before the third round ended.

Diaz connected with a left hand from long range in the first round, but Rivera fired back with a left hook that landed to the side of Diaz’s head.

The loss aside, Rivera enhanced his reputation by being so competitive against an undefeated southpaw in a fight for which he hadn’t been preparing prior to this week.

Rivera took the fight on just four days’ notice when Lara withdrew from the fight Tuesday after reportedly suffering a back injury. Rivera wasn’t among the 40 featherweight contenders the WBC listed on its website as of Tuesday night, but the Mexico City-based organization still sanctioned it as a 126-pound elimination match.

Despite the short notice, Rivera was in shape to box Saturday night because he had been training for a fight against the Philippines’ Drian Francisco (29-4-1, 22 KOs) on September 22 in Ontario, California.

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.