The U.S. Men’s Elite Cycling team may have lost its secret weapon for the World Championships next weekend.

American Tom Danielson of Garmin-Slipstream began today’s Vuelta a Espana 18th stage ninth on the general classification, 8:28 behind leader Alejandro Valverde of Caisse d’Esparge. Danielson crashed in Stage 17.

The cyclist told cyclingnews.com he may have broken a rib in the fall and that though he had been feeling on good form, he’d subsequently felt worse and worse due to a respiratory infection. He pulled out with 100km to go in today’s stage.

Danielson was scheduled to ride both the time trial and the road race at worlds. If he cannot return in two weeks for Worlds, USA Cycling will have to name a replacement to fill Danielson’s spot on the team. Top names George HIncapie (Columbia-HTC), Dave Zabriskie (Garmin) and Lance Armstrong (Astana) have verbally ended their 2009 seasons and both Hincapie (BMC Racing) and Armstrong (Team Radio Shack) will be riding for new outfits next year.

USA Cycling did not name a list of alternates.

The 2009 World Championships will be held in Sept. 23-27 in Mendrisio, Switzerland.