The Portugal star joined the Serie A champions on a four-year contract from Real Madrid after they agreed to pay €100m (£88m/$117m) plus €12m (£10.5/$14m) in additional fees.
The Bianconeri have been on a mission to win a first European crown since 1996 and have come close under Allegri, reaching the final in 2015 and 2017, losing to Madrid in the latter.
Juve were knocked out of the tournament by Real Madrid at the quarter-final stage last season. Ronaldo was the pivotal figure over the two legs with three goals and an assist as Madrid progressed on away goals after a 4-4 draw on aggregate.
Ronaldo and Co. went on to win a third consecutive European crown, but the talisman announced his imminent departure shortly afterwards.
And Allegri hopes Ronaldo's arrival in Turin tips the balance in Juve's favour as they aim for a clean sweep of trophies each season.
"I know he's very happy, he's a great professional and an important value for a team that has already done great things," he said at an event at the European Parliament. "I believe that the management and the president have done something extraordinary for Juventus, but above all for Italian football.
"Winning the Champions League was always a goal for the club and certainly the purchase of Cristiano Ronaldo gives us even more a push to achieve it, but this together with the league, the Italian Cup and the Super Cup, which are the four goals for which Juventus has always aimed."
There were suggestions earlier in the summer that Allegri could be heading the opposite way, as Madrid sought a successor for Zinedine Zidane after his surprise resignation.
And Allegri admits he had some decisions to make, but is happy to stay in Turin.
"I knew that this question would come up, but I only followed what I felt," he said. "This was definitely not an easy summer for me, because my mother passed away a month ago and I had to make different choices as well.
"I talked to Juventus and I'm fine here. I repeat my thanks to [Real Madrid president Florentino] Perez, I knew he had tried, but I had already given my word to Juventus, who had an important project."