Guardiola gestured to a supporter near the City bench to replace him as manager following a verbal confrontation.
He revealed afterwards it was because of his decision not to make any substitutions, the first time he has failed to make a single change in a Premier League game since May 2022.
"He asked me to make the substitutions, one player out and one in, and I asked him which one, I didn't know it," said Guardiola.
"I said 'come here and do it.' I saw that the team in that moment was still alive and sometimes to come in, the rhythm is difficult.
"I saw the team were still there and fighting for every ball. The managers, we have that option to make the substitutions but the managers decide or not."
Guardiola said ahead of the game against Newcastle he was worried after losing a number of key players, including Kevin De Bruyne, to injury.
However, Julián Álvarez scored in the first half to earn a 1-0 win to ensure City maintained their perfect start to the season with two wins from two games.
"This is the second game of the season but it is proof of the reason why we won a lot," said Guardiola.
"The mindset and mentality of this group of players amazes me every time, surprises me every time.
"Against that team and the conditions we arrive, with a lot of injuries and no rest, they played really well. How they defend and how they run and the commitment of everyone was absolutely amazing."