Pirlo had been out of action after injuring his calf against Borussia Dortmund, but made his return in Juve's 1-0 win over Monaco in the UEFA Champions League quarter-final first leg on Tuesday.
The influential midfielder played 74 minutes in Turin, with his lofted pass leading to Ricardo Carvalho's foul on Alvaro Morata, which allowed Arturo Vidal to net the winner from the penalty spot.
"Pirlo had a good 70 minutes, which is excellent considering he hadn't played for 50 days," Allegri told Sky Sport Italia.
"Set plays can be decisive in these games and I needed great quality on the passes to break through those tiny spaces, and in fact his pass was decisive for the penalty.
"Overall I think the result is deserved. Obviously, it's a quarter-final, so anyone who thought we'd win 3-0 was kidding themselves.
"They have the strongest defence in the Champions League and Ligue 1. The result is good, as we didn't concede on home turf and that is a small advantage.
"It will be different in Monaco, but they proved how organised they are in defence today with pace and technique in attack.
"I am always optimistic, but we know that we need to score in Monaco to go forward. This is a tough side to play against and it was easier against Borussia Dortmund because of their style of football.
"It's only normal there was pressure around this game, especially as we were the favourites, but we were too hasty and therefore made simple mistakes with our passing.
"In these situations, you must always have the patience and calm to play your game."
