The Italy midfielder has not played for the Serie A champions this season after sustaining a hip injury at the end of last month.
Allegri on Friday stated that the veteran playmaker trained with his team-mates on Thursday, but is not ready to play a part in Bergamo this weekend.
Pirlo, who has been linked with a move to LA Galaxy, will instead play in a friendly in an attempt to prove his fitness for either the UEFA Champions League clash with Atletico Madrid next Wednesday or the league showdown with Roma four days later.
Allegri said: "Andrea Pirlo worked with the team yesterday. His test results were good and he'll play a friendly on Sunday. Hopefully [he will be] back next week."
The Juve coach also gave a positive update on defender Andrea Barzagli, who has also been sidelined since undergoing heel surgery in July.
"Andrea Barzagli is undergoing an individual training programme with the aim of returning to the squad in a week's time," said Allegri.
Juve have won all four Serie A games so far this season and won their last eight games against Atalanta, but Allegri is braced for a tough examination this weekend.
"It's never simple for anyone to win in Bergamo. Atalanta approach the game on the back of two defeats and will cause us problems," he said.
"Atalanta are different from last year. They possess offensive wingers and have pace up front. We'll need to stay alert.
"We need to go there prepared and aim to claim three fundamental points ahead of Atletico and Roma. We'll immediately turn our attentions to Atletico after Saturday in Bergamo. One game at a time."
