Sabella believes that if the Barcelona forward were unavailable, it would be difficult for the team to come up with a plan to handle his absence.
"The thing is, there are times when Plan B is pretty similar to Plan A, and there are times when it isn't," Sabella told FIFA.com. "That's when you have to adapt.
"We've played a few games without Leo, but the fact is that he's irreplaceable. There's not a club or national team in the world that could play the same with or without Messi.
"We rely on him so much that when he's not there, we notice it. If it did happen, we'd have to see how we'd handle it and look at the match in question."
Argentina are bidding to win their third World Cup when they head to Brazil this summer.