Injury plagued Moody, a Lions tourist in 2005, hobbled out of England's first World Cup warm-up match against Wales with a ligament problem.
Moody, who also missed the entire Six Nations with a knee injury, was forced to sit out the remainer of England's preparations.
"We hoped he would be fit to start training but he is not quite there", said England head coach and Lions legend Martin Johnson.
Moody was initially confident of proving his fitness ahead of the clash with the Pumas but Johnson is unwilling to risk his skipper.
But he remains confident that Moody will play a major role in the remainder of the tournament. England face Georgia in their second game on September 18.
"He has been running around today so he is not a million miles away," Johnson added.
"It is just one of those calls, 'are you ready to participate fully this week and play a Test match this Saturday?' Not quite."