Buffon, who has suffered an injury-hit two years, was substituted at half time in Monday's 1-1 draw with Paraguay in their Group F opener in Cape Town having suffered the injury in the warm-up.
"Buffon has a strong sciatic problem. It is impossible to estimate when he might return," an Italy statement said.
He will undergo intensive treatment and his condition will be monitored daily given the unpredictable nature of such injuries.
The 32-year-old had suggested after the game on Monday that he might be able to return to training in two days and could be fit for Sunday's second group game against New Zealand.
"The coach substituted me because I had problems in my leg," said Buffon, who has 102 caps and was key to their 2006 triumph.
"I was not very good during the warm-up, my leg was hurting. I hope this injury will be better when we play New Zealand."
Buffon missed Tuesday's training session where Federico Marchetti, who replaced him at halftime against Paraguay for only his sixth cap, had an intense session of shotstopping.
Most of the players who started Monday's game were rested from training or went for a light jog while the remainder of the squad played a training match against a local youth side.
Midfielder Andrea Pirlo again sat out the session as he battles to recover from a calf injury which ruled him out of the Paraguay game and is likely to sideline him for the New Zealand clash too.
Italy's final group game is against Slovakia in Johannesburg on June 24.
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