The African champion defeated Bosnia-Herzegovina 1-0 in Cuiaba on Saturday to move second in the group, two points behind the Albiceleste, who have already qualified following their opening two victories.
Messi bagged a dramatic late winner during an uninspiring clash with Iran on Saturday, while he also scored the crucial second against Bosnia-Herzegovina.
"Messi is a normal human being, a football player," he told reporters. "Of course we have to strategize and make our plans to make sure we get things done [against Argentina]."
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Onazi was impressed with his side's performance in their surprise 1-0 win over Bosnia-Herzegovina and feels he carried out coach Stephen Keshi's instructions to the letter.
"We improved on our passes and our attacking and made sure we did everything as a team," the Lazio midfielder said. "The coach gave some instructions for me to do, and I had to do it. I'm the one in charge in the midfield and I had to calculate how to move forward, when to come back. It was the game plan for the team."
Next week's match against the Albiceleste will be the fourth time both teams have played at the World Cup, with the South Americans having won all three encounters, albeit narrowly.
After watching Iran hold Argentina for more than 90 minutes before Messi scored a late clincher, Onazi feels his side is ready to take on one of the pre-tournament favourites.
"We're taking every game step by step. No one gave us a chance and we are not ready to say much, we just want to go step by step," Onazi said.