MADRID, Spain — Spain's Davis Cup captain Albert Costa says Rafael Nadal "is worried" about his constant injuries.

"He is worried about his physical state, but he's working every day to get better," Costa said Wednesday.

Nadal will be sidelined for Spain's Davis Cup match against Switzerland next week. Nadal and top-ranked Roger Federer will miss the best-of-five series on March 5-7.

Another serious knee injury forced Nadal out of the Australian Open in January. Federer had announced earlier he would not play Davis Cup, and pulled out of the Dubai Championships because of a lung infection on Sunday.

Nadal has been plagued by injuries since November 2008, when tendinitis in his knees forced him to miss Spain's Davis Cup win over Argentina. The 23-year-old Nadal missed several months last summer because of knee problems after the French Open, where he failed to lift the trophy for the first time in five years.

"Until last year at Roland Garros and the problems, he had achieved everything," Costa said. "It's normal, after 22 years and winning everything, to have some down time."

Four-time Grand Slam winner Manolo Santana believes Nadal will recover and be back to his best in time to defend his title next month at the tournament in Indian Wells, Calif.

Nadal helped Spain win a second straight Davis Cup in December. But Costa said there is "no limit" to his team even without Nadal and Fernando Verdasco (leg injury).

Former top-ranked player Juan Carlos Ferrero, David Ferrer, Tommy Robredo and Marcel Granollers will represent Spain at the 11,000-capacity bullring in the northern city of Logrono.

"That was my decision, that was a technical decision," Costa said. "I think (the others) also deserve a chance to represent Spain. You have to have alternatives, so you're ready for any future tie also."