LAS VEGAS — Joseph Diaz Jr. and Jorge Lara promised fireworks in the Canelo-GGG co-feature Saturday, but those plans are now scrapped.

Lara withdrew from the bout Tuesday, just four days before he was slated for the biggest bout of his life: a WBC eliminator where the victor will earn a title shot against Gary Russell Jr.

The reason is unclear. Sources told RingTV.com that Lara cited a back injury suffered during training this week in Las Vegas, but that he actually was way overweight. Officially, Lara pulled out “due to reasons that have not been verified,” Golden Boy Promotions announced.

In steps Rafael Rivera, who already was in training for a fight next week, and ostensibly should be in fighting shape. He actually was slated to fight Diaz in this slot originally, but was passed up when it was decided Diaz would be fighting in a title eliminator, since Lara is the higher-rated boxer in the WBC’s rankings.

Rivera (25-0-2, 16 knockouts) owns a win over former champion Wilfredo Vazquez Jr., and with one more victory, will be in line to become a champion himself.

“Originally, I was hoping to get a shot at JoJo Diaz,” Rivera said. “When that didn’t pan out, I took another fight for next week. I have therefore been training and am in the best shape of my life. I am grateful that I have this last-minute chance, and there is no way I am letting this opportunity pass me by.”

Diaz, a 2012 Olympian from Southern California who is rated No. 7 by THE RING at featherweight, will be fighting on his biggest stage yet. He’ll also finally be fighting for something tangible: a title shot. The 24-year-old now must do with a last-minute change in opponent.

“I don’t care who is in front of me, I’m leaving with a win on September 16,” Diaz (24-0, 13 KOs) said. “After I have my hand raised in victory, I will be one fight away from realizing my dream of winning a world championship.”

Mike Coppinger is the Senior Writer for RingTV.com. Follow him on Twitter: @MikeCoppinger