United meet Nottingham Forest on Saturday night.
"That is very clear," Amorim said. "We are a massive club but we are not a massive team. We know it, so there is no problem to say it."
"We're not one of the best teams in the league. We have to say and think that clearly.
"But our past, our club is maybe the best one in the league. So here we have a problem. We have to focus on the little details, then we will improve as a team."
Amorim also warned his players about their fitness.
"If we want to win the Premier League, we have to run like mad dogs," said Amorim. "Even with the best starting XI on the planet, without running they will win nothing."
- Get breaking football news faster with the new Tribal Football app! Download now on iOS and Android to stay ahead of all the latest transfers and football updates. Download now: App Store, Google Play