Lionel Messi could sit out for Argentina

Scans have shown no tangible injury but the Barcelona forward has been training separately from the squad in the Georgetown University gym and appears unlikely to start the friendly in Washington, DC.

Tuesday's clash with Ecuador could be a more realistic target for Messi as coach Gerardo Martino considers selecting different line-ups for each game.

Argentina are also adapting to a new system after Martino replaced Alejandro Sabella following the World Cup, but midfielder Ever Banega is confident their understanding is developing well.

Banega told Argentinian newspaper Ole: "The system is new because with the other coach we played differently. But this one also is very good, the players are getting used to it."

The Albiceleste will go into the Copa looking for their first major tournament win in 22 years, a drought prolonged when they lost last summer's World Cup final to Germany.

Banega's midfield colleague Javier Mascherano told television channel TyC Sports: "It is difficult to feed off the past in a country that lives off the present.

"The sadness at losing the final of the World Cup will be always with me. We have aims in the national team and with our clubs. We feed off short-term goals and now we need to win the Copa America."

Albert Roca knows his El Salvador side face a daunting test, Messi or no Messi.

The coach told Salvadorean paper La Prensa Grafica: "Messi is an outstanding player, but if he is not able to play that would not change Argentina's potential.

"Everybody thinks that it is possible to mark Messi, but really he is something out of the ordinary and as with everything out of the ordinary, he causes problems for any tactical system.

"But we face a team where not only Messi is important. There are several big players, we have to be able to stop all of them.

"We know our own potential and the kind of challenge it is to face a national squad so talented. We have to face them with responsibility and commitment."