The 32-year-old - pivotal in Italy's World Cup victory in 2006 - has been in fantastic form this season and is one of the main reasons Juve are rejuvenated and battling it out with AC Milan for the title.
But ahead of Saturday's crunch clash between the top two at the San Siro, Allegri says he has no regrets at having let Pirlo leave.
"He's an amazing player and he's doing very well at Juventus," the Milan coach told a local radio station.
"Often in life after many years you need a change.
"In Turin he's found new motivation which after 10 years at Milan had decreased.
"But there's no discussing his quality."
Allegri also sprang to the defence of injury-prone Brazilian forward Alexandre Pato who has been unable to reproduce his best form since the coach took over at the club 18 months ago.
Asked if he'd been a disappointment, Allegri replied: "Absolutely not, unfortunately he's had two years of suffering various injuries.
"Now he's better and he's back in, although we rested him for precautionary reasons on Sunday.
"He's a great professional and he will be available on Saturday."
The coach also defended bad-tempered forward Zlatan Ibrahimovic, who will miss the crunch clash with Juve at the weekend as he serves the last of a three game suspension for slapping an opponent.
"He's like everyone else but the bottom line is he's a good lad, he's generous and he shows that in training.
"When he plays he always puts himself in the first line of defence for everything and everyone."
Allegri was referring to his previous defence of Ibrahimovic's actions when he claimed the big Swede had slapped Napoli's Salvatore Aronica to defend his team-mate Antonio Nocerino, who admittedly had been grabbed by the throat by the Napoli defender.
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