Gianluigi Buffon exited Italy's 1-1 draw with Paraguay on Monday at halftime. ((Filippo Monteforte/AFP/Getty Images) )

Italy goalkeeper Gianluigi Buffon has a "serious" back problem and the team physician said Tuesday it's uncertain whether he'll be able to play again in the World Cup.

Buffon exited Italy's 1-1 draw with Paraguay on Monday at halftime, and was replaced by backup keeper Federico Marchetti.

Team physician Enrico Castellacci said the problem was with the left side of Buffon's sciatic nerve, adding that "at the moment, no date can be set for his return."

After the game, Buffon said he thought he would back to normal in just a couple of days.

Castellacci also said Buffon, who was absent from Italy's training Monday, will immediately begin physiotherapy.

The goalkeeper's absence would be a severe blow to the defending world champion, which next faces New Zealand in Group F on Sunday.

Buffon was sidelined with injuries for several months this season, but declared himself ready just in time for another trip to soccer's marquee event.

The goalkeeper then thought he could withstand more pain when his back suddenly flared during the last warmup for the Paraguay match.

"I thought I tore something apart, but it turned out I just aggravated my sciatic nerve," Buffon said after the game.

He appeared his usual indomitable self before 62,869 fans at the Green Point Stadium until Antolin Alcaraz's header in the 39th minute left him motionless as the ball flew past him into the bottom right corner of the net.

Marchetti, who plays for the small Serie A club Cagliari, was never challenged in the second half as Italy rallied to tie the game a scrappy goal from Daniele De Rossi.