Ibrahimovic's goals provided the catalyst for the Rossoneri as they celebrated their first Scudetto since 2004 at the end of last season.
"Ibrahimovic was very important for Inter, but less so at Barcelona," Ancelotti told Sky Sport Italia.
"He has been a determining factor at Milan and has restored enthusiasm there."
Ancelotti's comments actually came prior to Milan taking the field on Wednesday night but his words rang true again as the Sweden striker inspired his team to a 2-0 UEFA Champions League win over Viktoria Plzen in Group H.
Ibrahimovic was central to Milan's win as he scored from the penalty spot and assisted the second for Antonio Cassano. After the game, Viktoria coach Pavel Vrba was left in no doubt that the return of Ibrahimovic from injury was vital for Milan.
"Ibrahimović was the best player on the pitch; it was lucky for Milan that he had recovered in time as he decided the game," Vrba told UEFA.com.
Ibrahimovic is expected to feature for Milan when they travel to Turin to play Serie A leaders Juventus on Sunday night.