Authorities were uncertain about his chances for recovery.
Horn suffered a serious injury to the left side of his neck and underwent surgery late Friday.
"The overwhelming likelihood is that we'll have to wait two or three days before we really understand the full extent of these injuries," Alan Feldman, an MGM Mirage spokesman, said Saturday.
Horn, along with longtime partner Siegfried Fischbacher, have been a Las Vegas Strip staple for years, performing their magic show to sold-out crowds at The Mirage hotel-casino. The shows, with their signature white tigers and lions, are among the best known and most expensive on the Strip.
"For more than four decades, I have had the great privilege of standing at the side of this remarkable man, and I will continue to do so during this very challenging time," Fischbacher said in a statement Saturday.
Halfway into Friday night's performance, Horn appeared alone on stage with the tiger and told the crowd the animal was making its debut in the show - a claim hotel officials said was part of the act.
The tiger, which weighs about 600 pounds, then lunged at Horn, who tried to beat the animal off with a microphone.
"I knew he was in trouble right away. I was horrified," said Diane Weightman, who was in the audience. "I wanted to jump on stage and help him. I didn't know what to do."
Andy Cushman, also in the audience, said Horn "looked like a rag doll" as the tiger dragged him off the stage.
Feldman said stage crew members used fire extinguishers to distract the animal and free Roy.
After the attack, Fischbacher appeared on stage and told the 1,500 audience members the performance was canceled, Cushman said.
Hotel officials said the show has been canceled indefinitely. The tiger involved in the attack, a 7-year-old male named Montecore, was quarantined at the hotel, officials said. Feldman said the animal had been used in the show for several years.
Horn, who turned 59 on Friday, had never been injured during a show before. "Not a scratch, not by an animal," said Bernie Yuman, the duo's longtime manager.
The illusionists signed a lifetime contract with The Mirage in 2001. The German-born pair perform six shows a week, 44 weeks per year and have been onstage in Las Vegas for more than 35 years.
Siegfried and Roy were at sea, working on an ocean liner, when they met in 1957. Siegfried performed the magic tricks, while Roy was his assistant. After suggesting they try to make a cheetah disappear, Roy smuggled one of the animals from a German zoo and used it as part of their act, according to the duo's official Web site.
The pair traveled around Europe, performing for five years before they got their break in a Monte Carlo casino. An agent in the audience, impressed by their show, invited them to Las Vegas. Siegfried and Roy made their debut at the Tropicana hotel-casino in 1967.