Ronaldo, 36, had one year left on his contract with Juve but returned to Old Trafford in August for €23 million.
Ronaldo's last game for Juventus came in the 2-2 draw at Udinese on the opening day of the Serie A season where his stoppage-time header was ruled out by VAR.
"Cristiano Ronaldo left on Aug. 28," Chiellini told DAZN. "This creates a shock and I think we paid him in terms of points in the start of the season.
"It would have been better had he [Ronaldo] left on Aug. 1 as we would have prepared ourselves and we would have arrived ready for the start of the campaign."
Ronaldo won five trophies, including two Serie A titles, in his three seasons at Juventus. He also scored 101 goals in 133 appearances.
After Ronaldo's departure, Juve picked up just one point from their next three league games and are sixth in the league table, 10 points adrift of league leaders Napoli.
However, Massimiliano Allegri's side are top of their Champions League group and three points clear of holders Chelsea after winning all three games.
"Cristiano needed new stimulus and a team that played for him," Chiellini added. "When he finds a team like that, he is always crucial and has always proved that even in his three years at Juve."
Chiellini also said that while Juventus are looking to the future, Ronaldo has joined a club that is "more focused on the present."
"A rejuvenation and restart program is underway here at Juventus," he said. "It is clear that if Ronaldo had stayed it would have been an added value and we would have gladly exploited it, but it could also have been the case that he decided to restart from a team more focused on the present rather than on the future."
Ronaldo, who spent six years at United from 2003 to 2009 in his first spell there, has impressed since returning to Old Trafford.
He has scored six goals in eight appearances in all competitions and bagged the winning goal in Wednesday's 3-2 Champions League win over Atalanta to go top of Group F.
The Portugal captain was named the Premier League's player of the month for September and is in contention for his sixth Ballon d'Or after being named among the 30 nominees for the prestigious award.