Biggest difference between Morata and Pereyra is age. Morata is practically a kid, so naturally he is uber pumped up, plus he is competing against his own national teammate who has an impressive first campaign with us. Imagine being 19,20,21 years old and signing for Juventus and the team bringing you knowing you will play, you want to make your mark as quickly as possible.
Pereyra doesn't get benched for not scoring, strikers do. There is more to Pereyra's game that can be analyzed by Allegri. But looking at Llorente, he can be doing other things well but if the goals aren't coming the red flags get raised. Tevez also needs someone upfront that he can combo with and Llorente isn't the most competent with the ball at his feet, perhaps Morata can add that to the attack?