Napoli come into the game having lost 2-0 at home to Atalanta on Saturday, their first league defeat for 15 matches.
Allegri says Juve, who had been the previous side to beat Napoli domestically in October, are prepared for a fierce reaction.
"We will face a Napoli who will be very angry about the defeat against Atalanta," he said at his pre-match media conference.
"It is not the case that in three days they have lost everything they have done well in these two years, so it will be a complicated game.
"We have the Coppa Italia and we must create the right conditions in order to reach the final, it would be our third final in a row, and then maybe we can try to win it.
"Above all it is cup semi-final, where the most important thing is not conceding goals, especially because we play the first match at home."
Allegri will review the shape of his players after their 2-0 win over Empoli last time out before deciding on his line-up.
"We have five defenders, who I will assess - someone will rest through necessity, then I will decide who to play," he said.
"I have to evaluate the whole team. Those who did not play Empoli had a good workout on Sunday. There will be some changes and I will assess what those will be.
"The most important thing is that the team is always tidy and does not lose strength. We hope to open up the game right away. Although there are 95 minutes to win games, you can get frustrated and lose faith if you don't do so right away."
The second leg does not take place until April.