BUENOS AIRES, Argentina (AP) -- Lionel Messi is probably under the greatest pressure of his career, having been blamed for Argentina's poor performances in its first two games at the Copa America.

His teammates, however, are coming to his defense and coach Sergio Batisa is expected to change his lineup for Monday's decisive match against Costa Rica, bringing in Gonzalo Higuain, Sergio Aguero and Angel Di Maria and taking out Carlos Tevez, Ezequiel Lavezzi and either Ever Banega or Esteban Cambiasso.

Messi's father, Jorge, said Friday his son was under "terrible pressure" and was upset when he and his teammates were booed off the field following Wednesday's 0-0 draw against Colombia.

"Leo is really down about this," his father told Radio 10. "He can't understand how this happened. It's the first time they've whistled at him."

Twice voted the world's best player, Messi has done very little in two games and his team has scored only one goal.

The Barcelona star was expected to help Argentina win its first major title in 18 years. Instead, the Gauchos must defeat what amounts to a second-string Costa Rica team Monday. A loss and Argentina will almost surely fail to make the knockout stage, unthinkable for the host country, which views itself among soccer's top half-dozen powers.

Costa Rica and Mexico are using mostly under-23 Olympic-style squads in this tournament because they are not part of the South American confederation and are playing as guests teams to boost the field to 12. Their full-strength teams played in the Gold Cup last month in the United States.

"Barcelona is another story," his father added. "It's a team that's been together for four years. The (Argentine) national team cannot match that. Playing in the Copa America at home in your own country is terrible pressure."

Messi walked through a light training session Friday, nursing a sore right ankle. He will play Monday, but he will have different teammates.

"I'm think of changing the system," Batista said in a Friday column for the newspaper Clarin. "Messi is going to be the playmaker and play very close to his forwards."

Barcelona and Argentina midfielder Javier Mascherano said it unfair to blame only Messi, who is a bit of a stranger in Argentina. He left his native country for Spain when he was 12, has never played for a popular Argentine club and has a reserved personality that leaves some Argentines cold.

"It's not Messi's fault we are not playing well," Mascherano said. "We're not playing well as a team."

"If after the game they (fans) insult you, they have a right to do it," he added. "We are aware of the situation, and it depends on us to turn it around. Beginning Monday we start another Copa America."

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