City were majestic in the first half of the first-leg clash, running into a 3-0 lead before half-time but eased off in the second half, with United eventually scoring a consolation goal through Marcus Rashford.
Guardiola believes that it was unrealistic to expect his team to win by a more comprehensive margin, despite the comprehensiveness of their first-half performance.
"We play United, what do you expect, a 7-0 win? In general it was an incredible performance at Old Trafford," he said after the match.
"The first minutes were tight but then we had an outstanding goal and we played really good. In the second half they changed their shape and we struggled a little bit to keep the ball.
"It's not over because it is United, they are able to score goals but hopefully we can reach the final again.
'I don't feel sorry for United' - Guardiola
"It is hard to control Rashford because he is so fast so we are comfortable with the ball so we played many passes."
City lost at home to United in the Premier League in December and Guardiola is not counting on a third straight final in the competition just yet and is wary of a stunning comeback.
"The last game (a 2-1 loss in December) we could not control when we lost the ball and tonight we did it better.
'Pride at stake after worst United showing of season' - Solskjaer
"We lost the ball in a position which is so dangerous (for the United goal). We cannot forget which team we play. Tonight we can be more than satisfied to come here to Old Trafford and win.
"It is not over. We have one more game and we'll see what happens. Of course Manchester United can come back. Last year here they lost to PSG and they qualify (in the Champions League). They have the shirt which means history and pride."