The Senegalese striker was signed by former Toffees boss Roberto Martinez from Lokomotiv Moscow for £13.5million in February this year but has been a massive flop since arriving at Goodison Park.
Niasse, who has appeared just seven times for the Merseyside club, scored his first Everton goal for the U23s earlier this week but still does not get a look-in under Koeman.
The Dutchman said: "He's still a contracted player at Everton, but I don't think that changes anything.
"I mentioned before the season started that he's a player who will not be in my plans but he's under contract and needs game-time for if there is interest from other clubs.
"Normally, we have enough strikers and different strikers to what he is. I prefer these people and not Niasse in the team."
Niasse had the chance to leave Everton in the summer but opted to stay put which was a decision which Koeman could not quite fathom.
"That's a question for the player. There was some interest in him but not the interest that he liked to have," he added.
"It is all up to the player whether he doesn't leave, I can't understand it, but that is normal.
"Football players like to play football, that's how I see it, and I explained to him why he was not in my plans for the season."