Far too early to make a judgement on Niasse, however it's fair to remark that there were some slightly disappointing signs yesterday.
Yes he was put in a difficult situation by the manager, yet you'd have liked (if not expected) to show some fight in what was a energetic game.
Instead he looked lethargic and well short of the required work-rate needed for the PL. Is that partly due to lack of fitness? I don't know.
He seemed unable or unwilling to tackle, which may be a fitness or culture thing coming from Russia, but not jumping for loose balls was shocking.
He'll certainly need to do a lot more when he's given a starting opportunity if he's to influence games as defences will not give him an easy ride.