I read something like Kean lost the ball 6 times when he came on and lost all of his duels. Niasse came on for him and didn't lose the ball and won half of his duels.
Problem for Kean, and especially when you are playing for a coach who is pure passion and fight like Duncan Ferguson, is that you have to become that old fashioned British centre forward in the gritty moments. As a 19 year old Italian he has no idea how to do that yet. That was Ferguson's mistake, Kean just became the fall guy.