The Italy captain was in inspired form during the first half of the game in Turin, playing a key role in a result which leaves the Nerazzurri 10 points behind third-placed Lazio
The struggling Nerazzurri had the better of a scrappy first half and forced the Italy captain into a string of fine saves.
The hosts then took over in the second period and opened the scoring when an unmarked Martin Caceres headed home Andrea Pirlo's corner, before sealing three precious points through substitute Alessandro Del Piero, leaving Ranieri cursing the brilliance of Buffon and a fatal lapse in concentration from his defence.
"That set-piece changed everything," the Inter boss said of the opening goal during his post-game interview with Premium Calcio.
"Until that, we had created so much and only a super goalkeeper like Buffon denied us a goal."
Ranieri elaborated on his frustration over Caceres' opener on Sky Sport Italia: "There is regret at having wasted a good hour of play by again conceding a goal after switching off for a set-piece.
"There was only a few in the penalty area but we were drawn to [Mirko] Vucinic at the front post, leaving only Caceres in the middle. Then we didn't react.
"I'm sorry because we could have come away from here with a different scoreline after that first half."
Inter's loss at Juventus Stadium leaves them eighth in Serie A, 10 points adrift of third-placed Lazio.
However, while Ranieri concedes that the Nerazzurri now have it all to do if they are to qualify for next season's Champions League, he refused to throw in the towel just yet.
"We must fight and take it one game at a time," the Roman reasoned. "Then we'll see where we are.
"Sure, to win here would have given us a big push but this year we have not been able to get on the right track.
"Ten points to make up on Lazio is a lot, it is true, but we will not give up."
Inter's next Serie A outing comes against Genoa at the Giuseppe Meazza next weekend.
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