The £125m is struggling with form and fitness in his first weeks as a Liverpool player, which has coincided with the Reds losing four consecutive games in the Premier League.
“Fans are critical, and there's a lot of criticism flying around because Alexander Isak is not performing," Waddle told BoyleSports.
"You have to realise, he handled it all wrong at Newcastle. He said he wanted to leave, refused to train, and went to Real Sociedad to train basically on his own.
"He didn't play any friendlies, then went to Liverpool and suddenly he's in the Premier League which is a hard and physical league, mentally as well.
“He hasn't hit the ground running at all. He's probably two or three weeks behind most players. We know he's a talented footballer and he will probably come good. I'm not saying he won't, but he handled it all wrong and he's paying the consequences now for being behind.
“He hasn't done well with Sweden either. Sweden never qualify for the World Cup despite some of the players they have, and they are bottom of their group. It's just unbelievable."