Spain captain Emilio Sanchez Vicario called on the 56th-ranked Granollers on Tuesday to fill in for the top-ranked Nadal, who is out of the Nov. 21-23 final because of a knee injury.
The annoucement came after Tommy Robredo ruled himself out as a possible replacement for Nadal, saying it would be "impossible to be available."
While the 22-year-old Granollers will make his Davis Cup debut, Sanchez Vicario filled out the rest of the squad with expected players.
The final will be played on an indoor hard court at Mar del Plata.
Robredo has been a regular for Spain since 2004, but he hasn't been picked since winning both of his singles matches in a first-round victory over Peru earlier this year.
Since winning at Bastad in July, Robredo has reached the quarterfinals only once and crashed out of the Paris Masters in the second round.
"I haven't even gone out running for five minutes and I wouldn't be able to defend my country as I would like to," said the 21st-ranked Robredo, who sees Argentina as the overwhelming favorite to win its first Davis Cup trophy.