Nacional host the first leg on Wednesday (0115 BST Thursday) at the Defensores del Chaco in Asuncion where Piatti has permission from his new MLS club Montreal Impact to play for the Argentine favourites.
However, the Saints have tried but so far failed to get that permission extended for one more week for the second leg at the Nuevo Gasometro in Buenos Aires after the Impact said they wanted Piatti in Montreal on Friday, Argentine media reported.
South America’s top club competition was interrupted for two months because of the World Cup finals in Brazil. The quarter-finals were played in mid-May and the semi-finals not until mid-July.
Brazil’s woes were not confined to a disappointing World Cup for the national team. They also failed to get a team into the last four of the Libertadores Cup for the first time since 1991.
Holders Atletico Mineiro, who were the fourth Brazilian side in a row to win the competition last year, were knocked out by Colombia’s Atletico Nacional in the round of 16.
Nacional are a small Paraguayan club side who had never before progressed beyond the group phase and have only one foreign player, Argentine-born goalkeeper Ignacio Don who has spent the better part of his career across the border and taken out Paraguayan citizenship.
“We know this is an historic chance for the club and each one of us. It’s something new for us, we’re beginners, we’d never passed the group phase … and now we’re among the two best (teams),” coach Gustavo Morinigo told the Argentine sports daily Ole.
San Lorenzo and Nacional, both first time finalists, were the two teams with the worst records in the group phase who qualified for the round of 16.
The Paraguayans then upset Argentine former winners and favourites Velez Sarsfield, while the Saints beat Brazilian sides Gremio and Cruzeiro, both twice winners of the competition, in successive ties.
“That tie (against Velez) represented a lot for us. There was a before and after. We thought they were the hardest team we’d meet,” said Morinigo, whose side then eliminated Argentina’s Arsenal in the last eight and Defensor Sporting of Uruguay in the semi-finals.
San Lorenzo have right back Julio Buffarini available again after he missed the semi-final second leg against Bolivar through suspension having scored in the 5-0 rout of the Bolivians in the first leg. The Saints won the tie 5-1 on aggregate.
Veteran playmaker Leandro Romagnoli, another player leaving San Lorenzo after the final to join Brazilian first division side Bahia, said: “This is my World Cup final.”
Coach Edgardo Bauza, who has already won the title in 2008 when in charge of Ecuador’s LDU Quito, said he was recently contacted by the Argentine FA as a prospective successor to Argentina’s World Cup coach Alejandro Sabella, but was not letting himself be distracted.
“I’m not losing any sleep over that,” Bauza said. “Right now my head is exploding as I try to make the right choices and no mistakes (for the final).
“We don’t feel we are favourites, they (Nacional) want to put that pressure on us, but the team who play better will win,” said left back Emmanuel Mas, who scored twice against Bolivar.
The trophy winners will go through to the Club World Cup in Morocco in December.
(Writing by Rex Gowar; editing by Justin Palmer)