Caceres, who has played for the Bianconeri in two previous spells, arrived in Turin on Saturday to undergo a medical ahead of a reported loan switch from Lazio.
Allegri confirmed at the weekend that Caceres would be looked at as a replacement for Medhi Benatia, who has since completed a move to Al-Duhail, and the Uruguay international could be involved right away.
The Juve coach announced on Tuesday that Giorgio Chiellini, Daniele Rugani and Joao Cancelo will all start against Atalanta, with the left-back role still to be decided. Andrea Barzagli and Leonardo Bonucci are both out.
Caceres is not set to make the XI, but Allegri said of the versatile defender: "Benatia has officially left. Caceres is completing his transfer and, if all goes well, he could be available."
@OfficialAllegri: "Let's start with the injuries: @Miralem_Pjanic and @SamiKhedira are both available, maybe even @MarioMandzukic9. @Cuadrado, @andreabarzagli2 and @bonucci_leo19 are all out, although Leo's injury is less serious than expected."#AtalantaJuve #CoppaItalia— JuventusFC (@juventusfcen) January 29, 2019
"I am always hearing about Atletico," Allegri told reporters. "It is a football match, it is not confirmed that Juventus are the clear favourite.
"We talk as if Juve are just waiting until June 1 to win the Champions League. This is crazy stuff and not a good way to think about the Champions League. It is our goal to win it and, if we are good and we are lucky, we can progress.
"Playing against Atletico is not like playing against any other team, it is a game where there may be unexpected events that become normal.
"In the league, it's different because there are 38 games and you can miss one. [In the Coppa or Champions League,] there is no opportunity to respond. You have to play a great game and be lucky, there's nothing else.
"The Champions League is to come, but we do not need to think about it every day."