Bayern took the lead after just 25 seconds when Buffon, 35, failed to stop David Alaba's long-range effort.
"How was he positioned? He seemed like a pensioner," said Beckenbauer.
"I don't want to upset him, but he should have kept that shot out. Perhaps Gigi didn't think that Alaba would shoot from so far.
"Bayern were focused from the first until the last minute and Juventus hardly had a shot on goal. The result is tight for us, we should have won by three or four goals."