He said: "We lost 2-3".
City paid for their lack of ruthlessness in front of goal when United scored three times in the second half to dash the Citizens' hopes of lifting the title on Saturday.
Two goals in the space of 97 seconds early in the second half brought United level after Vincent Kompany and Ilkay Gundogan gave City a commanding 2-0 lead at the interval.
And after the fixture which cruelly postponed City's celebrations, Guardiola offered insight into what he told the enigmatic character as they were pictured having a lengthy conversation after the final whistle.More news: Diego Simeone: "Atletico Madrid fans should be proud"
However, City's tendency to pay heavily for momentary lapses in concentration is a recurring theme of Guardiola's career. We had a good game in general, especially of course the first half.
"So hopefully it will not be too much damage until next Tuesday. The schedule is like this, that's why this competition is so tough", Guardiola said.
"But a good manager (Jose Mourinho) maybe said in the first half "guys, we have to do something more".
"But at that level against teams in the Champions League, against United the boxes made the difference, and today we missed lot".More news: Blitzboks claim bronze in Hong Kong
"Of course you always have bad moments, and we've had bad moments that last two games".
The damage would have been greater had Raheem Sterling not spurned a pair of glorious chances but City's wastefulness came back to haunt them as Chris Smalling volleyed a 69th-minute victor. So that's not too much. Now we will try, if we are fearless enough, to stand up again and focus on Tuesday. "I have to think about it".
Man City still lead the Premier League by 13 points, but their path to the title is not all plain sailing, as their next six games include Spurs, and five teams battling for survival at the bottom of the table.
Guardiola's side trail Liverpool 3-0 at the halfway stage of their quarter-final tie and he admitted the collapse against United could play on their minds tomorrow.More news: Apartment fire breaks out in Trump Tower leaves 1 dead