Medical tests Tuesday confirmed the tendinitis, and Spain team doctor Angel Ruiz-Cotorro said the top-ranked player would undergo treatment through Saturday.
Nadal will have more tests Monday to assess the injury. Spain plays Argentina in the final on indoor hard court at Mar del Plata from Nov. 21-23.
Spanish Davis Cup captain Emilio Sanchez Vicario will not pick Nadal if he is unfit, Nadal's uncle and coach Toni said Tuesday.
On Monday, Nadal pulled out of the season-ending Masters Cup in Shanghai citing fatigue after a season in which he's played in 111 matches.
Nadal, winner of a fourth straight French Open title and his first Wimbledon championship, withdrew from the Paris Masters on Friday with a knee problem after losing the first set of his quarterfinal match against Nikolai Davydenko.
Nadal also won the Olympic gold medal in a year when he supplanted Roger Federer at No. 1. Without Nadal, Spain would be regarded as an underdog to win its third Davis Cup trophy in eight years.
Spain beat the United States 4-1 in the semifinals, with Nadal and David Ferrer playing singles and Feliciano Lopez and Fernando Verdasco the doubles.
Injured Nadal doubtful for Davis Cup final
Usain Bolt Is A Blanco
