Gianluigi Buffon doubt for Juventus' decisive UCL clash with Olympiakos

Gianluigi Buffon is a doubt for Juventus' decisive Champions League clash with Olympiakos on Tuesday night with a late fitness test required to convince coach Massimiliano Allegri his captain can play.

Buffon, 39, did not train fully on Monday due to a calf strain and Wojciech Szczesny is on alert to make his Champions League debut for the Bianconeri as Allegri faces another enforced change to his defence.

"There are only two problems I need to look at: one is Buffon, the other is [Miralem] Pjanic," said Allegri, who is already deprived of the injured Giorgio Chiellini, at a news conference in Piraeus. "If they are fit, they will play.

"At the back, with Chiellini out, either [Daniele] Rugani or [Mattia] De Sciglio will start. [Gonzalo] Higuain will play because he's physically and mentally fit, and we've got [Mario] Mandzukic available again."

Juventus will seal second spot in Group D if they win, but a draw or a defeat would see them exit the competition if Sporting beat Barcelona.

Allegri said he considers Tuesday's game to be the most important of Juve's season so far -- more so than Friday night's Serie A trip to Napoli, which they won 1-0 to reduce the deficit on the Azzurri to a single point, while they are two adrift of the new league leaders Inter Milan.

"It's far more difficult and far more important than our game at Napoli because this is our first decisive game of the season," he said. "In the league, there is time to recover but tomorrow, either we win or we risk going out, so it's complicated.

"Champions League games are never easy to win, particularly away from home, and so it will be complicated. We've got to show great respect, without considering what could happen in Barcelona."

The Bianconeri have successfully navigated the group stage of the Champions League in each of Allegri's first three seasons in charge, reaching the final twice, and the former AC Milan coach said: "When you reach certain levels, the big games give you adrenaline and emotions and they're beautiful to play.

"Up to March, the league is not going to be decided, unless a team loses seven in a row. Big games like these -- like Napoli, Olympiakos, Inter, Roma, then in the Coppa Italia -- are the ones you look forward to most.

"The Napoli performance and result were important for our objective to be in the running come March to win the league. It was a beautiful night in Naples, but that's got to become normality -- against Olympiakos and then on Saturday against Inter."

Juve host Inter on Saturday night while Roma will pay the Juventus Stadium a visit on Dec. 23.