Juventus lost its 49-match unbeaten streak in Turin, and the veteran goalkeeper was quick to question the performance of his colleagues, while complimenting Inter's conviction.
The Bianconeri went ahead in the first minute through Arturo Vidal's strike, but three second-half goals saw the reigning champions beaten 3-1 in Turin, as Inter moved to within a point of the league leaders.
"I did not like our attitude after we went 1-0 up," Buffon told reporters. "I don't understand why we dropped so deep, we invited Inter onto us. Inter had more hunger to win the game, and they deserved the victory. We had forgotten what it felt like to lose, and it is not pleasant."
The 34-year-old then admitted Andrea Stramaccioni's men are genuine title contenders and compared their display to that of his own side last season.
"They have improved since last season, and with their coach they are becoming title opponents to us and Napoli," Buffon said. "They interpreted the game well, and with great conviction. So much so, that they reminded me of Juventus last season.
"After the first 15 minutes we gave them the initiative, and they knew what they had to do to take three points."