Sesko made his debut as a second-half substitute in defeat to Arsenal last week.
"When you are at a club like this you want the best players, but you also have to think about the future. I like to watch players on video and gather all the information, but I felt that Sesko was even better than I had thought," Amorim told Sky Sports.
"I have to be careful. Sesko will be our striker for many years, so we have to get it into his head, let him land and enjoy the day."
On their major spending this summer, the Portuguese also said: "Success means winning titles, but we don't know if we'll get there. We're not in a position where we can say we're fighting for the league. We have to take small steps and prepare every game like a final.
"Everyone knows what the goal is. Not for this season, but in three years we will win the Premier League."