Former Manchester United captain Keane, speaking on Sky Sports, said Haaland's "general play" was "so poor" and "almost like a League Two player".
Haaland came under fire from Keane for his lacklustre performance in City's goalless draw against Premier League title rivals Arsenal on Sunday.
Haaland is the Premier League's leading scorer this term with 18 goals after winning last season's Golden Boot during a remarkable first season with City.
He netted 52 times in all competitions as Guardiola's men lifted the Premier League, Champions League and FA Cup.
Keane conceded the 23-year-old Norwegian is in a class of his own in front of goal.
But City boss Guardiola was unimpressed by Keane's remarks, insisting any blame for the scoreless display against Arsenal should be shared by the team.
"I don't agree with him, absolutely not. He's the best striker in the world and he helped us to win what we won last season. Erling is exceptional," Guardiola told reporters on Tuesday.
"The reason we don't create chances is not because of Erling. The standards that Erling has are unbelievable and everyone expects other things.
"There are games where he could be better but I knew during the game against Arsenal that the reason why we were not creating chances was not because of Erling.
"We needed more presence in the box. If we had that Erling would have more space. It happens sometimes. If you want to score goals, bring a lot of players to the opponent keeper, it is as simple as that."
City's failure to beat Arsenal has damaged their bid to win an unprecedented fourth successive English title.
They are three points behind leaders Liverpool ahead of Aston Villa's visit to the Etihad Stadium on Wednesday.
Guardiola will need Haaland at his predatory best if City are to extend their reign as champions.
And the Spaniard accepts that criticism comes with the territory of being a high-profile player.
"If you don't want to accept that as a football player, you have to dedicate yourself to another job," Guardiola said.
"When you are a public figure, you have to accept it. That's why when you are in contract talks, you have to ask for a lot of money to accept those moments.
"I'm surprised it comes from former players. With journalists I can understand because they have never been on the pitch but the former players is always a surprise (when they are critical)."
