The 41-year-old American pulled out of the Volta a Catalunya two weeks ago, revealing via Twitter that he had 'tweaked something on the steep climbs in Tirreno.' In a press release, the RadioShack-Leopard team said Horner was suffering with suffering with an iliotibial band friction syndrome.
He traveled to Basel in Switzerland last week for treatment and is now recovering at home in Bend, Oregon. He is expected to be back in training next week and is likely to focus on being at his best for the Amgen Tour of California in May.
Horner won the 2010 edition of the Vuelta al Pais Vasco and was second in 2011. In his absence, Andreas Kloden, Maxime Monfort and Haimar Zubeldia will lead the RadioShack-Leopard team in the six-day race.
Andy Schleck continues his slow comeback from his 2012 injury and personal problems by riding the Vuelta al Pais Vasco. Completing the RadioShack-Leopard team are Matthew Busche, Laurent Didier, Ben Hermans and Jens Voigt.
Kim Andersen and Josu Larrazabal will share directeur sportif duties for the race that begins on Monday with a 156km road stage around the town of Elgoibar.