The Spanish midfielder was given his marching orders barely a minute into the second half following an altercation with Morgan Gibbs-White.
Rodri grabbed Gibbs-White around the throat following a coming together, leaving referee Anthony Taylor with no option but to show him a straight red card.
Rodri is now set to serve a three-game ban which will see him miss games against Arsenal, Wolverhampton Wanderers and Brighton and Hove Albion.
Despite Rodri's dismissal, Manchester City held on to win 2-0 and preserve their 100% start to the season.
Nonetheless Guardiola says he is angry at his in-form midfielder, who failed to keep his emotions in check but also criticised referee Taylor, who also dished out ten yellow cards during the game, excluding one for the former Barcelona and Bayern Munich boss.
"The game was incredibly perfect for 35 minutes for both sides but after that, when it became chaos, it was not our responsibility, that is for sure," Guardiola said.
"With all the yellow cards, the responsibility is not Nottingham Forest or City's that's for sure. The referee has changed the game, absolutely."
"Hopefully Rodri will learn," Guardiola added. "He had to control himself and his emotions. I can get a yellow card, but Rodri can't. I don't play - the guys who do have to be careful.
"Kyle has done this in the past - they know they shouldn't do it - but it is emotions. Whatever happens, you have to control yourself. I am angry, because I don't like playing with 10 men.
"I said to the players at half-time to be careful because of what had happened in the last 10 minutes [of the first half] but we fell into the trap."