He also said that one of the reasons for including him in the squad is our failure to bring in another striker.
Sounds like he wasn't in his plans but circumstances have forced him to include Niasse.
Watching Niasse play, you can see why Koeman doesn't like him. His technique is poor. A few times against Chelsea U23 he was running down the line and managed to dribble the ball out of play. But he is still quite effective. He is a real nuisance to defenders with his pressing. He's got quite good reaction times and speed over a few metres, when there's a loose ball he often pounces on it first. And he also has a decent finish on him. I wouldn't want him starting, but he's not a bad option to bring on against a tired defence.