Barca recorded the greatest comeback in Champions League history last week, recovering from a four-goal first-leg deficit at Camp Nou to win the return match 6-1.
Juventus are 2-0 in front going into Tuesday's last-16 second leg in Turin, but Allegri is feeling far from secure.
"It will not be easy," Allegri, who will make a late decision on defender Giorgio Chiellini's involvement after he was named in the squad, said at his pre-match media conference.
"Everybody thought that Barcelona were already out and now they are favourites. It's normal, things can change really quickly in football, we just need to be there among the best eight and then try to give our very best.
"There are a lot of risks, because first and foremost it’s a game of football and a game of football can never be discounted.
"Let's not forget what Porto did in their Champions League play-off away to Roma this season [a 3-0 win] – they were starting that match at a disadvantage. This will be a tough test."
He added: "They are a team who know how to play these kind of games and they know very well how to take advantage of their chances.
"So we need to reduce it to one game, without thinking about the 2-0 lead, but by thinking and doing the things which will bring us to another home win."
Allegri is not interested in sending a message to Juve's rivals to win the competition and simply wants to get the job done.
He continued: "Qualification is the only thing that counts on Tuesday, we don't have to send any signals to our rivals, we just need to qualify and play a good game. Fireworks are used in the final – they don't matter here.
"We have an important objective to achieve, we want to reach the quarter-finals. The spirit is certainly good but we have to be aware that we need another great performance in order to qualify.
"But we do need to believe we are capable of winning the Champions league. It is a magical competition."