Ronaldo was given the full 90 minutes by Ancelotti, who saw second-half goals from Andrea Pirlo and Alberto Gilardino cancel out Robert Acquafresca fourth minute opener for Cagliari.
Ancelotti said afterwards: "We did well not to fall apart despite conceding that early goal, hitting the woodwork and seeing a penalty saved. The run of games that take us up to the derby has started positively.
"After four months out, Ronaldo did his duty. Kaka was tired."