Messi is out of action for six to eight weeks because of a hamstring injury sustained in Barcelona's 4-1 win over Real Betis last Sunday.
Argentina played out a goalless draw with South American rivals Ecuador in Messi's absence on Friday, and will continue their preparations for next year's World Cup in Brazil when they face Bosnia-Herzegovina in St. Louis on Tuesday.
The 26-year-old - who has struggled with injury problems for much of 2013 - has scored 39 goals in 78 appearances for Argentina.
And Sabella admits that his side would be a very different proposition at Brazil 2014 without the considerable talents of the four-time world player of the year.
"I think we are a world power. I think Brazil, Germany and Spain are the favourites for the World Cup... Messi is the best and if he is not there, not only with the national team but all teams, everyone is going to feel that," Sabella said.
"With or without Messi, it makes a huge difference."
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