Juventus -- runners-up twice in the last three seasons -- head to Athens needing a win to ensure their passage to the knockout phase.
But Gonzalo Higuain will play after scoring the winner against Napoli at the weekend despite a broken hand.
"Higuain will be on the pitch tomorrow, because he is doing well both physically and mentally," said Allegri.
"Tomorrow could be decisive. We are still evaluating Buffon and Pjanic, who is suffering from a muscle strain."
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"We can't afford to wait and see what Barcelona's result will be," continued Allegri.
"Either we win or we run the risk of exiting the Champions League. Tomorrow (Tuesday), our approach must be spot on. We have to be patient and take our chances to win the game."
"With (Giorgio) Chiellini out, either (Daniele) Rugani or (Mattia) De Sciglio will play. (Mario) Mandzukic will be available as well."
Allegri added that it would be an important test for Argentina striker Paulo Dybala "to return to his highest level" and score his first Champions League goal this season to add to his 12 in Serie A.
"We understand the importance of the game and we know that it is not going to be easy for us -- the first match against Olympiakos at home was complicated and now we are playing in their stadium.
"We have to be focused and careful tomorrow because it is a complicated game.
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