How so? It's not like Morata had always Tevez and Llorente to fight for a regular spot. With Dybala, Zaza and Mandžukić he played full 90 mins only in 5 games for the whole season. It's not realistic to project his last year form in a new team that doesn't have even the half of the creative product Modric, Kroos, Marcelo, Ronaldo etc can put to the table.
Rashford last year scored the decisive goal against Anderlecht, goal and assist against Celta in the SF, 5 in the league including 1 against the champions Chelsea. Looking at the two Rashford strikes me as the better prospect/potential if we project what they both have done/did so far in their careers.
Its just a predictably myopic and intellectually disingenuous line of thought. "If player X wasn't a star at club Y then why would we buy him" ? Thank heavens managers look into these matters with a bit more nuance than that.