The Brazil striker pulled a hamstring towards the end of the previous campaign and has yet to feature in pre-season.
The 30-year-old Ronaldo will miss Thursday's friendly at Real Betis although attacking colleague Alberto Gilardino is back after knee surgery.
"Ronaldo will be available for the first match of the championship," Ancelotti told a news conference dominated by questions about Brazilians.
New signing Alexandre Pato, who according to media reports cost 22 million euros (15 million pounds) from Internacional, cannot feature for Milan until January under the terms of the deal but Ancelotti has already been impressed by the 17-year-old forward.
"In January he will be able to play immediately," he said. "In theory he could play with us today. He could calmly demonstrate even in this team the characteristics he has already shown."
Ancelotti also denied fresh speculation in the Italian and Spanish media linking the European champions with Barcelona playmaker Ronaldinho.
"Ronaldinho? Talking about him in April could have made sense but today, almost in the middle of August, it seems to me it is a 'no' really," he said.
(Writing by Mark Meadows in Rome)
