RIO DE JANEIRO, Brazil (Reuters) -- Teenage striker Alexandre Pato is on a lengthy Internacional injury list as the Libertadores Cup champions begin their title defense away to Uruguay's Nacional on Wednesday.

The 17-year-old, nicknamed "The Duck" has been out of action with an ankle injury since helping Brazil win the South American under-20 championship in January and, at best, is likely to make only a substitute's appearance in the Group Four match in Montevideo.

Defenders Cesar and Indio and midfielder Fabian Vargas are also doubtful for the Brazilians and along with key forward Fernandao, who is recovering from a back operation.

It will the first competitive match for Inter's first team since it beat European champions Barcelona to win the Club World championship in Japan in December.

Inter coach Abel Braga has fielded the reserve team in the regional Gaucho championship as he concentrates on defending the Libertadores.

Nacional, three-time former champion, is attempting to restore some pride for Uruguay, whose clubs have performed poorly in recent tournaments.

The group, which also contains Argentina's Velez Sarsfield and Ecuador's Emelec, is regarded as the toughest of the eight in the first round. Velez beat Emelec 1-0 away in the group's opening match last week.

The week's other top fixture is in Santiago on Thursday where Chilean champions Colo Colo host Daniel Passarella's River Plate of Argentina in a meeting of two of the title contenders.

The teams have been drawn in Group Six along with Caracas FC of Venezuela and LDU of Ecuador, which kicks off the group in Caracas the same evening.

The week's 10 games also feature two Peru-Mexico clashes with Alianza Lima hosting Necaxa in Group Two on Tuesday and Cienciano taking on Toluca at high altitude in Cuzco in Group Seven on Wednesday.

Apart from Internacional, three more of Brazil's six representatives are in action.

Flamengo and Parana host UA Maracaibo (Venezuela) and Real Potosi (Bolivia) respectively in Group Five on Wednesday while Vanderlei Luxemburgo's Santos makes the long journey to Colombia to face Deportivo Pasto in Group Eight.

Flamengo and Parana both made good starts away from home last week, Flamengo holding Potosi 2-2 at nearly 13,000 feet above sea level last Wednesday and Parana beating Maracaibo 4-2 on Thursday.

Flamengo's game means the players were given only Sunday to enjoy Rio de Janeiro's annual carnival celebrations before being ordered back to training on Monday.

Parana only arrived back home on Sunday, its return trip having taken it via Caracas, Bogota and Sao Paulo.

Potosi also faced huge logistical problems to reach its game, beginning with a 170-km bus journey from to Sucre followed by a flight to Santa Cruz on Monday.

After an overnight stay, they are due to fly to Sao Paulo on Tuesday, take a bus across to city to another airport and then fly to Curitiba.

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