The Norwegian marksman brought up his tally to 19 goals in all competitions this season with a Champions League brace in a 5-0 win over the Danish side on Wednesday.
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Haaland, who struck in the seventh and 32nd minute, has now banged in 28 goals in the Champions League from just 22 games.
Incredibly, the Norway international has found the back of the net in every City fixture since August 13 - a run of nine consecutive matches.
His brutal finishing reduced teammate Grealish to laughter at the Etihad Stadium, with Haaland taken off at half-time before he had the chance to register his third hat-trick this term.
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And the England man revealed that Grabara - the former Liverpool goalkeeper - spoke to him during the game as he was left dumbfounded by the 22-year-old.
Speaking to BT Sport about Haaland after the rout, Grealish said: "It's unbelievable, I have never witnessed anything like it in my life.
"For the first or second goal, I was just laughing because I was just like… 'how?'
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"He is always there and is always finishing it. He is a pleasure to play with at the moment.
"When I was walking back after one of his goals their 'keeper said something to me and I was like 'what?'
"He said, 'This guy is not human'. I was like, 'Bro, I know, you're telling me!'
"He is unbelievable and is a pleasure to play with.
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"He is so humble, he's great around the place and that isn't just me bigging him up, that is the God's honest truth and hopefully he can carry on this form and carry us to glory."
It's not just Grealish who has been left speechless by Haaland's devastating form, but also his boss Pep Guardiola.
Cameras picked up the Spanish gaffer' priceless reaction after the ex-Borussia Dortmund star doubled City's lead.
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Commenting on his reason to hook Haaland at the break, Guardiola said: "He plays a lot of minutes, the game was under control and Cole Palmer is an excellent player and was looking forward to playing.
"If the game would have been tight Erling would have continued but it is better to rest for Southampton - who we weren't able to beat last season.
"It's not about resting players. We have a squad of 20 players if you just play 11 you don't have 20 players. The other ones, when you need them, don't work."