Goals in each half from Jhon Duran and Morgan Rogers were enough to seal Villa’s win, with a late consolation from Man City’s Phil Foden not enough for Guardiola’s floundering champions.
Pressure is now mounting on Guardiola as he struggles to dig City out of a slump that has seen them lose six of their last eight Premier League matches.
Haaland's goals have also dried up, but he insisted the City dressing room is united behind Guardiola.
"He won the Premier League six times in seven years, so we will never forget that,” said Haaland.
“He will find the solutions. He has been doing that every single year. We still believe in him, we have to work harder than ever right now.
"We know how important confidence is and you can see that it affects every human being. That is how it is, we have to continue and stay positive even though it is difficult.
"Of course we are disappointed, it's not good enough. It's not good enough from me."
Guardiola insisted his side’s fortunes will turn, as he pointed to absent players once again to explain their slide in form.
"Step by step, we have nice personalities in the team and sooner or later we are going to find it," stated the City manager.
"We played a really good first half, and the second half we dropped, we were not good. The tempo as the game goes on is a bit more difficult. Congratulations Aston Villa on the victory.
"Of course there are more reasons (for City’s dip in form). We concede the goals we don't concede in the past, we don't score the goals we score in the past. Football is not just one reason. There are a lot of little factors.
"Aston Villa are a team that are doing well in the Champions League, Premier League as well. It is a tough place to come. Last season we won the Premier League, but we came here and lost.
"We have to think positive and I have incredible trust in the guys. Some of them have incredible pride and desire to do it. We have to find a way, step by step, sooner or later to find a way back."