Paulo Dybala scored after just 44 seconds, with a Filip Bradaric own-goal accounting for Juventus' second after 39 minutes, two minutes after Joao Pedro had pulled the Sardinians level.
Ronaldo failed to add to his seven-goal league tally for new club Juventus with the Portuguese superstar hitting the woodwork before next week's Champions League visit by former club Manchester United.
But the 33-year-old's pass across goal set up Colombian Juan Cuadrado for his first of the season three minutes from time in the Allianz Stadium.
"He has always been a leader and an extraordinary player, as you saw from the assist he put on Cuadrado's foot," said Allegri.
"These are different players who in the decisive moment always come up with a goal or a great pass."
Juventus have 31 points from 11 games with Inter and Napoli both on 25 points.
It is the best ever start to a season for the Turin giants who have drawn just one game against Genoa.
Earlier, Inter Milan crushed Genoa 5-0 to reclaim second spot on goal difference ahead of Napoli, who had eased past Empoli 5-1 on Friday, and keep the pressure on the seven-time reigning champions.
But Allegri insisted: "This isn't good enough.
"We have to defend better, we concede too many goals and risked letting in an equaliser in the 85th minute.
"Even after the first goal we didn't add a second and drifted out of the game. We can't always assume that we'll score two or three goals."
Wins by the top three were a boost before next week's Champions League fixtures, with Inter Milan at home against Barcelona and Napoli hosting Paris Saint-Germain.
Eusebio Di Francesco's Roma, however, needed a late Alessandro Florenzi volley to snatch a 1-1 draw at Fiorentina ahead of their midweek trip to CSKA Moscow.
Allegri gave Douglas Costa his second start this season up front alongside Dybala and Ronaldo, who was presented with a special jersey before the match after last week becoming the first player to break the 400-goal mark in Europe's top five leagues.
The champions were soon off the mark as Rodrigo Bentancur weaved his way through for Dybala, who beat two defenders and the offside trap, squeezing the ball in despite a slip.
Cagliari pushed forward with Artur Ionita setting up Leonardo Pavoletti but Wojciech Szczesny cleared, before Pedro broke through.
Juve were soon back in command when Bradaric accidently turned a low Costa cross into his own net from close range.
But without Croatian Mario Mandzukic, who was left out, the leaders had difficulty pegging back the visitors.
Ronaldo rattled the post just before the first-half whistle. But the Portuguese forward surged forward with a counterattack from a corner to roll across for substitute Cuadrado to finish off.
In Milan, Luciano Spalletti's Inter notched up their seventh straight Serie A win on front of 70,000 spectators.
Roberto Gagliardini scored in either half (14, 49), with Portuguese midfielder Joao Mario getting the fourth (90+1) and setting up three others, including Matteo Politano (16) and late substitute Radja Naonggolan (90+4) who headed in as he returned from an ankle injury.
In Florence, Florenzi snatched a point for Roma five minutes from time to cancel out Jordan Veretout's first-half penalty.
In a clash between two teams fighting for European places Fiorentina, in sixth, stay just ahead of Roma on goal difference.
Roma had the best of the first half with a Federico Fazio header going dangerously close and Edin Dzeko firing wide after 20 minutes.
But Fiorentina took the lead after a penalty awarded for a light challenge by Roma goalkeeper Robin Olsen on Giovanni Simeone after half an hour.
Di Francesco's side continued to push forward with Lorenzo Pellegrini hitting the woodwork and Dzeko again unlucky before Florenzi volleyed in the equaliser.AFP