Just two points separate the top three in a fascinating Premier League title race as City moved to within a point of leaders Liverpool, with third-placed Arsenal one point further back with 13 games to go.
Haaland was keen to make amends for a frustrating night in front of goal as City were held 1-1 at home by Chelsea on Saturday.
The Norwegian has only recently returned from a two-month injury layoff and Guardiola said he was also coping with the loss of his grandmother off the field.
But Haaland was more like his old self in front of goal when he raced onto Julian Alvarez's pass to slot in his 22nd goal of the season 19 minutes from time.
"Top strikers score a lot of goals. Don’t criticise, he will shut your mouth. Sooner or later he is there," said Guardiola.
"He was out for two months, he lost his grandmother, it is not easy for a human being.
"We spoke about the moment (missing chances against Chelsea), and I realised this later, but he didn’t say anything about the passing away of his grandmother."
Kevin De Bruyne was left on the bench for the entire match as Guardiola revealed he has "niggles" in his hamstring.
The Belgian missed the first five months of the season due to a hamstring tear.
De Bruyne's ingenuity was missed as City struggled to make their domination count despite enjoying nearly 70 percent possession and having 25 shots on goal.
The Bees were the last visiting side to win at the Etihad 14 months ago and had their chances to spring another huge surprise.
Frank Onyeka failed to beat Ederson when one-on-one midway through the first half before Ivan Toney sent a dipping free-kick inches over the bar.
The vast majority of goalmouth action came at the other end.
Oscar Bobb came closest to opening the scoring in the first half, on a rare start for the Norwegian youngster, when his effort was cleared off the line by Ben Mee.
Bernardo Silva also headed a glorious chance wide as Thomas Frank's men held out until the break.
Liverpool, Arsenal hopes dashed
Liverpool and Arsenal's hopes of a second unlikely gift from City in the title race in four days would have been raised by a quiet start to the second period.
However, City remain well in the hunt to become the first side ever to win four consecutive English top-flight titles for a reason.
One slip by Brentford defender Kristoffer Ajer provided the moment Haaland had been waiting for as he galloped clear on goal and kept his composure to score for just the third time in his last eight games.
"I’m incredibly proud. We didn't give much away until the goal – which is a slip," said Brentford boss Frank.
"I think they’re good enough. They don’t need that luck to get over the line!"
City should have extended their advantage, especially as goal difference could yet prove decisive in the title race.
Phil Foden scored a hat-trick when the sides met earlier this month in City's 3-1 win in west London.
But the England international was denied twice late on by Flekken as the goalkeeper tipped over a blistering shot and then stood up well to block as Foden tried to go round him.
