Juve need four points from the final five games after closest rivals Napoli, 11 points behind, saw off Cagliari 3-2 in dramatic fashion at the Stadio San Paolo.
A disappointing Milan side now have to think seriously about a resurgent Fiorentina, who beat Torino 4-3, and now sit just one point behind them in fourth.
The result left Fiorentina five points ahead of Inter Milan, who got back to winning ways with a 1-0 defeat of Parma, veteran striker Tommaso Rocchi's 82nd-minute strike lifting some of the pressure on coach Andrea Stramaccioni.
"Milan are Milan, they always play to win," he told reporters. "It wasn't easy for us, if we lost to them, Napoli would have been closer and some pressure would have built up.
"I wouldn't say today was a decisive step but was a great step forward. To find ourselves 18 points ahead of Milan is a rare thing indeed. Was it a penalty? Without any doubt."
"We conceded very little to Juventus, but then we created very little ourselves," Allegri said.
"There are 18 points between us and Juventus. There's really not a lot you can say when you're up against a side like them, who could reach 90 points this season.
"This team still needs to grow, but in the meantime we need to think only about finishing third, it will still be a great achievement."
Insigne, the 21-year-old striker, fired in from just outside the area in the fourth minute of stoppage time to cement Napoli's claims on an automatic qualification place for next season's Champions League.
