Harry Kane made his return for Spurs after almost a month out with injury but it was Eriksen who orchestrated the victory with two goals in 11 second-half minutes, taking his goal tally to five in three games in all competitions.
"Because what we have done in this season, in the first half and moments in the second half, I am happy (with)".
Guardiola then added: "You have to defend a little better in the second half, but it is football but".
But United hit back with Paul Pogba scoring twice and Chris Smalling weighing in with the victor.
Losing to United would upset fans, but is unlikely to be detrimental to City's season.
And they were even more porous as United fought back to turn the game around in a 16-minute spell just after half-time as a Paul Pogba double and Chris Smalling's victor handed United a famous win.
"A special day for United which is a pity", Guardiola told BBC Radio 5Live.
Pep Guardiola's side dominated the first-half and raced into a 2-0 lead via goals from captain Vincent Kompany and midfielder Ilkay Gündogan.
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Yet just when it seemed United were being outplayed to the point of embarrassment, they launched an astonishing comeback soon after the break with Pogba, previously anonymous despite a new flashy blue hair-do, striking twice in two minutes.
However sweet it would have been to become champions with its greatest rivals - and record 20-time English champions - on the same pitch, City's priority is conquering the continent.
This season, that will require overturning a 3-0 deficit to Liverpool here on Tuesday.
Guardiola won two Champions Leagues as Barcelona coach, but despite all his domestic success in Germany, his Bayern Munich were side were knocked out in the semi-finals three times in a row. The lineup sent out to face United made that clear.
Lingering doubts over Aguero mean it is likely Gabriel Jesus will have a role to play.
They weren't missed, in the first half at least. "You explained about that is going to happen and it happened", said the Catalan coach. The loss to Liverpool at Anfield in the league didn't particularly mean much in the grand scheme of things for City, but it should have been a lesson for him for this game and again, his tactical blunder will see his team go out early in Europe again. "You know how it goes between City and United, you know the history, so it would be historic".
Sane crossed low as Jose Mourinho's side again struggled to cope with City's passing, only for Sterling to scuff his shot into the hands of De Gea, while Gundogan headed at the keeper before the visitors' Romelu Lukaku was booked for a foul. "No one was in the dressing room dancing around and celebrating after the halftime result".
He said: "We lost 2-3".