Lots of fans seem to get caught up in a conspiracy theory about him being a fraud. His position is surely also related to the assault incident back in may and him not being required through injury/suspension in first team. It's because he's behind Lukaku for front position that chances are so limited, Koeman has a system first and foremost and that doesn't have 2 strikers. Koeman may have wanted him out, but he also may have wanted him to help in reserves to help, develop, and be able to come into the first team hungry and enthused.
Whatever Koeman's thoughts upon arrival, I think it's difficult not to be impressed with how Niasse has got on with the disappointment and rejection. He's not sulking or only turning up when it suits him, he's doing the hard graft, off the ball, in the under 23s. But more than that, it appears that he's a well liked and regarded part of the team, in training as well as match day. He's doing all the right things in terms of demonstrating his worth and that will surely only go unacknowledged by Koeman for so long, probably until he's needed.
I think Koeman's position is also in the hope to be proved wrong, to create adversity and maybe some humility. If you want a more genuine appraisal, you only need to look at Unsworth's comments about Niasse, he understands the situatio, his talent and is managing him very well. He's a hard working and relatively unselfish player for a front man, he gets a surprising amount of assists, it's often more utilising a team mate in better shooting position than creativity, but with his pace and workrate he creates the space to get those opportunities.
Bolasie is now out for the much of the rest of the season and we might need some reinforcements. Long term, we won't keep Kone or Valencia and Lukaku rarely loses sight of the door. I think he was always considered a bit of a rough diamond and maybe he just needed time to improve technique and become a more well rounded player as well as adapt.
It's hard to know how things progress from here, he could carry on scoring goals in reserves and not get a chance. But I think there is also a fair to decent chance that he will either play himself into the first team on performances or when opportunity arises. I think he could well feature in some cup games alongside some others from under 23s. I also think there is a real chance he could explode into first team and prominence, it's his kind of style.