"I think we have enough time to improve in the upcoming games and before the World Cup, we will have a month to prepare and leave everything in place for the start", he said.
"It'll be hard for some to stay on".
Messi along with his Argentine teammates have failed to end La Albiceleste's drought, losing in two successive Copa America finals - in 2015 and 2016, and also finishing runners to Germany at the 2014 World Cup in Brazil.
Lionel Messi has defended Gonzalo Higuain, claiming the Juventus striker is one of the best in the world and deserves to be in the Argentina squad.
Messi said: "Mascherano? I'm not going to tell you what he told me, but I'll understand if he leaves, he wants to play more".More news: Arrest Made 7 Years After NBA Player Lorenzen Wright's Murder
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However, speaking ahead of City's Champions League clash with Shakhtar Donetsk on Wednesday, Guardiola, who oversaw the bulk of Messi's rise to becoming arguably the greatest player of all time, rejected talk that a transfer was ever on the cards.
"Being on the bench is disgusting".
According to Marca, City also offered the 30-year-old a mouth-watering £850,000-a-week to reunite with former boss Pep Guardiola.
Last Friday, Argentina, Nigeria, Iceland and Croatia were drawn together in World Cup Group D.
Sampaoli has a wealth of attacking options having called up Mauro Icardi, Paulo Dybala and Sergio Aguero alongside Messi last time out, but the Barcelona star feels Higuain is crucial to the national team.