The Norway striker has become known as being temperamental.
But Grealish told talkSPORT: "He is simply incredible, a 'killer'. This season he has scored 27 goals in the Premier League and there was a bit of a lack of respect, he was talked about as if he wasn't good.
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"He's incredible, he's there every time the ball enters the box. I can never speak highly enough of him as he deserves.
"We get along really well, he's a brilliant guy, he's so nice to have around. I don't think I've ever seen anyone have his work ethic. The things he does, both to help sleep, to recover. I've never seen nothing like it. This shows why he is so successful...".