If it's the case that it isn't right to dismiss a player after one season in a new league, then why don't we have Niassé as our leading striker?
What?
I genuinely have no idea if that is a real question?
In case it is,
Niasse got 4 in 17 for Hull last season. With the same strike rate, he would have got 9 goals. For a newly promoted team, that's a decent return. Hence why, despite him not even getting a look-in with us, he is coveted by other teams. Teams at the bottom of the league often rely on counter-attacking via direct play, especially against the top 10. Niasse has prove he can be effective in that kind of team, illustrated by his goal against the RS last season.
I would imagine the reason he isn't out first choice striker is because:
A) we aren't a team near the bottom of the league
B) we have cash
C) we have others who play the way he does but offer more (he's hardly a target man so he is up against Sandro comparatively)
No-one in here can say, "Niasse will never be a success" because success in itself is subjective. It's for the manager to decide whether, all things considered, the player is one he wants.
So if he wants Slimani because of x, y and z, it's nuts to suggest that based on his first season in the PL and all the other mitigating factors surrounding last year, that it's a bad buy because he has 'proved that he can't handle the PL'. At worst he's unproven and needs further time / games to fully ascertain whether or not this league / Leicester / We are the team to get the best from him. There are lots of success stories for strikers who had underwhelming first seasons, as i've mentioned above.