The 39-year-old has made a considerable impact since rejoining Milan in January, scoring 11 goals in 20 appearances in the second half of the 2019-20 campaign.
He has been in equally impressive form this season despite missing almost a month after contracting Covid-19, taking his tally to four goals in just two games with a double against Inter at the weekend.
One of the best strikers of his generation, Ibrahimovic spent two years with Milan earlier in his distinguished career, a stint that included securing the Serie A title in 2011.
Ronaldo, meanwhile, has been just as prolific since he joined Juve from Real Madrid in 2018, scoring 68 goals in 91 appearances across all competitions.
Zaccheroni, though, is in no doubt which player has had more influence on Serie A.
"I have coached great champions in my career, from [Oliver] Bierhoff to [George] Weah up to Adriano, but the only regret I have is that I never trained Ibrahimovic," he told Il Giornale.
"He is eternal and I still don't understand why he never won the Ballon d'Or.
"In Italy, he has shifted the balance more than Ronaldo. It is no coincidence that many young players have grown exponentially since his arrival.
"He doesn't just score. He transmits confidence to all his team-mates, carrying the team on his shoulders in difficult moments."
Ibrahimovic himself feels that his current form is unstoppable for opposition defenders and that his all-round game is being perfected with age.
"I feel more complete, I have more experience. When you are young you are still growing,” the veteran forward told Sky Italia after sinking Inter with a brace.
“You spend a moment where you are learning things, playing games, you have moments and situations that repeat themselves.
“Now I feel complete. If I had the condition of 20-30 years, nobody would stop me. But anyway, they don't stop me even now.”
Milan currently sit on top of Serie A with a 100 per cent record though their first four games - two points ahead of surprise package Sassuolo.