According to a team release, Horner 41, has "iliotibial band friction syndrome" on his left knee from the end of Tirreno-Adriactico where he was sixth overall.
"So far the doctors have been optimistic about my knee problem, but as soon as I started training, the pain came back," Horner said. "The last time that I was on the bike for more than four hours was in Catalunya. There is no way I can race now. All of my condition is gone. After Catalunya I was off the bike for four weeks."
Horner is set to consult a specialist in Arizona this week.
"It is possible I have an alignment problem. We will examine all possibilities. I want to race again. All of the doctors I have visited are optimistic that it will heal again, but it just needs time. The problem is that nobody is able to tell me how much time I need. This is not a collar bone fracture-like injury, where you can say exactly how much time you need to recover. My injury is more difficult. I can ride my bike but after two hours the pain returns."
"This is such bad news for me," continued Horner. "As the 2011 winner of this race I want to be there. Moreover, I live in San Diego, just 20 minutes from the team hotel. I will visit the team, but it will be with mixed feelings. It all hurts as I was so good in the early season. In Tirreno-Adriatico I was as good as the best riders in the world."
Horner is hopeful of returning for the Tour de Suisse and race the Tour de France but while he will not be able to lead the team in California, RadioShack Leopard has a solid second option in Matthew Busche who last year finished 14th overall.
"We will try to get him on the podium," said team director Alain Gallopin. "Furthermore I am convinced we will see something from Andy Schleck. He continues his comeback and is motivated and ready to show something. Haimar Zubeldia, sixth in last year's Tour de France, will also be in the front the coming week and with riders like Bob Jungels we can try to win a stage. If Peter Sagan will give us that opportunity..."
RadioShack Nissan for Tour of California: Matthew Busche, Laurent Didier, Markel Irizar, Bob Jungels, Ben King, Andy Schleck, Jens Voigt and Haimar Zubeldia.