The Turin giants go into the encounter at the Juventus arena as one of just three teams unbeaten in the league.
Last weekend's 1-1 draw at Catania lifted Antonio Conte's side level on eight points with Udinese at the top of the standings, with Milan three points behind them.
"This game will have more significance for us than it will be for Milan,'' Buffon told Gazzetta dello Sport.
"Milan is the reference team this season, they are the defending champions.
"This game will give us a strong indication as to where we are at in terms of our growth.
"We've enjoyed a good start to the season but it's early days and we have to just focus on ourselves and continue to improve.''
AC Milan travel to Turin in buoyant mood.
Wednesday's 2-0 triumph over Viktoria Plzen handed the Rossoneri their first Champions League win this season.
Massimiliano Allegri's side will be bolstered by the presence of Zlatan Ibrahimovic on Sunday.
The Swedish striker returned to play after a two-week absence through injury in midweek and scored a penalty and set up a goal for Antonio Cassano.
And Ibrahimovic warned his former club Juve his best is yet to come.
"I feel much better but I'm not yet 100%,'' Ibrahimovic said. "But I hope I will be come Sunday.''
Ibrahimovic will again partner Cassano up front.
Conte will have all of his players available with Montenegrin forward Mirko Vucinic returning from suspension.
Andrea Pirlo will play alongside Claudio Marchisio in the heart of Juve's midfield in what will be first game against his former club since leaving the San Siro outfit this summer.