The club is in a mess right now though Esk. Performance levels are very much tied to the psychology of the players and I can´t imagine he is exactly enjoying his move so far.
The toxic fans that @orly mentions don´t really help either
The club being in a mess as nothing to do with it whatsover; yes, confidence may affect your decision making and a stray pass may go a miss. But the lad misses all the basics of a footballer, awareness, touch, movement etc etc.
All the flops that have come to the PL, including people like Bily, still have the very basics that make a footballer; they can control a ball, they have awareness of what is around them, they can see space and know to run into. Niasse does not seem to have any of these, he's ran into people, hes ran ahead of a ball that was played to him.....its quite shocking.