City's Fabian Delph was sent off in that game at the DW Stadium and against Swansea the same player was shown a yellow card which led to a penalty, slotted home by Matt Grimes after 20 minutes.
Two goals behind to Championship also-rans Swansea, for 68 minutes nothing went Manchester City's way.
City had levelled in contentious circumstances too when Cameron Carter-Vickers was judged to have tripped replacement Raheem Sterling in the area and Aguero's penalty hit the post before rebounding in off Nordfeldt for an own goal.
"If it is not penalty, and it is offside like the people say, I'm sorry", Guardiola said.
The Red Devils were handed a red card following a reckless challenge by Victor Lindelof on 83 minutes, but the Video Assistant Referee overturned decision and the defender was only handed a yellow card. They ask all the clubs about the VAR and all around Europe, the VAR is there and here is not.
"It's centimetres. Sometimes the VAR takes three minutes to make a decision so imagine taking that decision in one second during the game".
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"But, at the same time, we played incredibly well, with the amount of actions, the amount of crosses and the goalkeeper (Nordfeldt) was incredible", he added.
"I'm sorry it was offside".
However, the reigning Premier League champions realised the uphill task they now face when Bersant Celina fired in Swansea's second from a brilliant team-based move.
"The penalty was really lucky but you need it, especially when you're in four competitions".
"We always believed it would come, always we fight". But for a long time tonight it looked like that might happen.
Guardiola was less impressed with the Swansea fans, who invaded the pitch following the final whistle with substitute Oleksandr Zinchenko appearing to be jostled by one supporter.