Not only did Terry concede Spurs' opening penalty, he then proceeded to earn himself an early bath in the second half.
Two fouls in quick succession left Chelsea man light for the rest of the game and he will now miss their home game with Stoke due to suspension.
Terry has been one of Chelsea's leading lights this season, but Italian boss Ancelotti has conceded that the England international was below par at White Hart Lane.
"I was a player and you know sometimes you are not always rational, you sometimes lose your control," said Ancelotti. "He has had a fantastic season.
"Maybe sometimes he had some problems and didn't play so well, maybe against Spurs he didn't play too well."
The defeat allowed Manchester United to close within a point of the league leaders and Ancelotti now wants everyone to try and remain calm despite the added pressure.
"Now we have to win the next game and if possible the next three games - then we are the champions," he continued.
"Tottenham played better than us, it can happen, but it's a time to stay calm."