Guerrero hurt the shoulder while sparring at his training site in Big Bear on Wednesday but initially hoped he'd be able to recover in time for the Aug. 27 fight in San Jose, near where he was born in Gilroy.
But the injury was worse Thursday morning and he had to pull out of the fight.
"It was numb and he couldn't move it at all. They iced it. They were hoping it was going to get better overnight. It did not. It got worse," Golden Boy Promotions CEO Richard Schaefer said. "He can't even put a jacket on or a shirt and move the arm at all. They are afraid it could be a torn rotator cuff. We don't know."
Guerrero was flying back to the Bay Area on Thursday to undergo an MRI on the shoulder. Schaefer said Guerrero's manager, Bob Santos, sounded extremely concerned about the injury when he notified Golden Boy on Thursday morning about the cancellation.
"Our thoughts are with Robert," Schaefer said. "This is a big setback for him. I talked to him yesterday morning. They were feeling great. He had a fantastic training camp. I heard Robert was in the best shape of his life. It's really too bad this fight will not happen."
Guerrero, the WBA and WBO Interim Lightweight champion, was moving up in weight to fight for Maidana's WBA Super Lightweight title. The fight was set to be nationally televised on HBO and was being held in Guerrero's native Bay Area.
Schaefer said he hopes that if the injury is not as serious as initially feared that the fight can be rescheduled for later this year. If that's the case, Golden Boy would look to keep it in San Jose.
"That's a fight which belongs there," Schaefer said. "We were right when we put it there. I looked at the ticket reports yesterday and they were extremely strong. I had no doubts we would have had a sold-out venue there on fight night. I hope it will be rescheduled because it clearly was one of those fights everyone was looking forward to."
Schaefer said he does not know yet what will be next for Maidana. The injury happened too late to find a replacement for the Aug. 27 card so now Maidana will wait and see how serious Guerrero's injury is before deciding whether to find another opponent.
"Marcos Maidana is a fighter that fight fans can't get enough of and would love to see him," Schaefer said. "He's really one of the most exciting fighters in the sport at any weight class. It really depends on what's going to happen with Guerrero. I would assume based on my conversations I had with Bob, I'd be surprised if Robert would not be out for quite a while. If that's the case, Marcos Maidana will be fighting someone else."
The heavyweight fight between Seth Mitchell and Mike Mollo that was on the undercard has also been canceled.
