Updated: July 17, 2008, 10:47 AM ET

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Welterweight Oscar Diaz, who collapsed in his corner after the 10th round of a TKO loss to Delvin Rodriguez on Wednesday night, remained in critical condition Thursday after undergoing surgery overnight to relieve swelling on his brain at San Antonio's University Medical Center.

"Oscar came through surgery well," Ron Katz of Rodriguez promoter Star Boxing told ESPN.com after talking to Donna Duva Brooks of Hall of Fame Promotions, Diaz's promoter, who was at the hospital.

"They had to remove the left side of his cranium in order to help the swelling go down, which apparently was very bad," Katz said. "He had a subdural hematoma [bleeding on the brain], which was taken care of. During surgery, he apparently showed movement in his arms and his brain pressure was very good, according to the doctor."

Diaz was under heavy sedation following the operation, but was not in a medically induced coma, Katz said.

Fighting in front of his hometown fans at San Antonio's Municipal Auditorium, Diaz appeared to be trailing badly in the fight. His right eye was swollen almost completely shut in the main event of the ESPN2-televised "Wednesday Night Fights."

Just before the 11th round began, Diaz, who was standing in his corner receiving instructions from trainer Tommy Brooks, buckled and passed out. Medical personnel came to his side immediately and within a few minutes he was on a stretcher and being taken to a waiting ambulance.

"When he left the ring, he was completely stationary and he didn't look good," Star Boxing chief Joe DeGuardia said. "They couldn't get him to respond. They cleared a path right away and took him to the hospital."

It had been a grueling, physical fight between Diaz (26-3, 12 KOs), 25, and Rodriguez (23-2-1, 14 KOs), 28, of Danbury, Conn. That kind of fight was nothing new for Diaz, a brawler who has been in several hard fights, none more difficult than an 11th-round TKO loss to Golden Johnson on Nov. 10, 2006.

Diaz took enormous punishment in that fight before it was stopped. He returned eight months later to face Juan Manuel Buendia, but the fight ended in a first-round no contest because of an accidental head butt.

Diaz then won a unanimous eight-round decision against Roberto Valenzuela in May to set up the fight with Rodriguez for the vacant regional USBA 147-pound title.