The 2006 World Cup winner, still widely regarded as one of the best keepers in the world, has a small herniated disc after suffering a sciatic nerve problem before Monday's 1-1 draw with Paraguay in Italy's Group F opener.
The 32-year-old is definitely out of Sunday's game against New Zealand, will almost certainly not play in the final group match with Slovakia on June 24 and could miss much more.
"Certainly on Sunday he can't be on the field," Italy team doctor Enrico Castellacci told reporters.
"We have decided he will stay with us and we will try everything for him to recover. When talking about a herniated disc, even small, we can't say how long it will take."
INJURY-PRONE
Injury-prone Buffon, who has suffered a similar problem in the past, was substituted at half time on Monday having first indicated his discomfort in the pre-match warm-up.
Inexperienced keeper Federico Marchetti, who replaced Buffon on Monday, is now poised to start on Sunday in Nelspruit for only his seventh cap.
Marchetti was playing in Italy's second division just two years ago.
Castellacci said Italy had already asked FIFA for permission to give Buffon special medication to treat the problem which although small can cause intense pain.
Italy centre-back Giorgio Chiellini bemoaned the loss of Juventus team mate Buffon but was confident Marchetti was ready.
"It's obvious how much he's worth to the team, and playing with him all season I know how important the World Cup is to him," he told reporters.
"Marchetti did a good job when he came on. He's got a lot of quality and he won't have any problem replacing Gigi.
"Goalkeepers live in their own world, so I think the best person to help him out is Gigi. The only thing I can provide is tranquillity. He doesn't need my advice."
Midfielder Andrea Pirlo, who missed the opener with a calf problem and will skip the New Zealand game as well, is on course to possibly be available for the Slovakia clash.
The rest of the squad trained normally in warmer conditions after Tuesday's biting cold.
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