Five-time Ballon d'Or winner Ronaldo finalised his return to United last Wednesday (AEST) for an initial fee of £12.9million (€15m), signing a two-year contract with the option of a third.
That brought an end to the Portugal captain's three-season stay at the Allianz Stadium, during which time he scored 101 goals across 134 matches in all competitions.
Ronaldo's stint in Turin was rather more mixed on the trophy front as Juventus won the Scudetto in his first two seasons but missed out to Inter in 2020-2021, despite the 36 year-old's league-high 29 goals.
That made Ronaldo, who last week became the all-time leading scorer in men's international football, the first ever player to finish as top scorer in Serie A, LaLiga and the Premier League.
Juve also fell short in the Champions League each time, including a shock last-16 exit to Porto last season, which ultimately played a part in the end-of-season sacking of Andrea Pirlo.
The Bianconeri have since reappointed Massimiliano Allegri, and Buffon – who spent two seasons with Ronaldo at Juve – believes the ex-Real Madrid player's decision to take on another challenge makes sense.
"I don't think the fans should be surprised," he told Radio anch'io. "He has the reputation of a great professional who rightly thinks about himself a lot.
"In these three years he has contributed with great performances and scored many goals. I don't see anything illogical in his choice to leave, he thought a lot about it.
"Juve are in a transition period. You pay a bit when these things happen. When you have a coach like Allegri, maybe you don't reach the target, but you get close to it."
Ronaldo was not the only high-profile name to depart Serie A during the most recent transfer window, with Gianluigi Donnarumma swapping Milan for Paris Saint-Germain.
Donnarumma missed just five league games for Milan over the past five campaigns, establishing himself as one of the world's best goalkeepers, but he has yet to feature for his new club.
The Italy international was late returning to training after being given an extended break on the back of Italy's successful Euro 2020 campaign and is now playing second fiddle to Keylor Navas.
Legendary Italian keeper Buffon, who spent a solitary season at PSG between spells with Juve, can understand why Donnarumma decided to leave boyhood club Milan.
"The choices of a lad, who is also a professional, must be respected," he said. "After years in which he has not played at certain stages, he has chosen to go.
"We are talking about a boy who has years and years left in his career, looking for the gratifications that I found.
"Paris Saint-Germain have one of their strengths in goal and that's why Keylor Navas is playing. I don't think Gigio will have problems being a reference."