On Thursday, Juventus ran out easy winners at home in the last 16 tie, after going behind in the opening minute, and now travel to face bottom side Salernitana again on Sunday.
"Playing in Salerno is never easy, we will find a difficult environment because they will want to avenge Thursday's result," Allegri said.
"Above all, by returning to league action, they will try to give continuity to their victory in Verona (on December 30th) which boosted their efforts to stay up in Serie A."
Inter Milan's 2-1 win over Hellas Verona on Saturday leaves second-placed Juventus five points adrift of the leaders going into the last game of the first half of the season, but Allegri insisted he was still looking over his shoulder.
"It will be a very complicated match and the boys know it," he said. "We must try to close the first round in the best possible way and then we'll start thinking about the second round.
"It's fundamental to maintain the gap on fifth in the standings," Allegri added, referring to Juve's 11-point advantage have over fifth-placed Bologna.
"I repeat it this time too - our first objective must be to qualify for the next edition of the Champions League and to do so we must continue to work well."
Allegri also gave some indication of who will feature against Salernitana.
"(Midfielder) Hans Nicolussi Caviglia will play tomorrow. (Left-back) Andrea Cambiaso, however, is feverish and unlikely to be there," the manager said.
"As for (striker) Moise Kean, he has not yet returned to training on the pitch, so it will still take some time before we have him back available.
"Gleison Bremer will return and I think Federico Gatti and Danilo will be starters. Timothy Weah could start, but I haven't decided yet. Whoever is better will definitely play, I will choose the best formation possible."
Juventus have a seven-point advantage over AC Milan in third, while Salernitana have 12 points from 18 games and are two points Cagliari who are in 17th place in the safety zone.
