The midfielder was sent off in the 46th minute of our 2-0 victory over Steve Cooper's side after an altercation with Morgan Gibbs-White.
We then held our own in the second half, restricting the visitors to just three shots on target as we made it six wins from six in this season's Premier League.
"He has done it. He has apologised. [He said] Sorry," he said.
"Rodri has to control his emotions. I can get a yellow card but Rodri cannot get a red.
"I don’t play, the players in the pitch have to control their emotions."
The Etihad crowd got behind 10-man City in the second half to help see us over the line for our 300th Premier League home win.
Guardiola believes that atmosphere was crucial as his players worked tirelessly to keep Forest out.
"You can lie to one person but not almost 60,000 people," he said.
"It’s impossible. They see we are not lazy, they see we do everything to defend a result from the difficulties we had in the second half.
"The spectators are involved in that and that’s why they can give more energy when they are tired."