Sesko, signed from RB Leipzig in August for a transfer fee of 76.5 million euros with a further 8.5 million in bonuses, has scored just two goals in 11 appearances for United.
Asked about the Slovenian's return, Amorim told reporters: "I'm relaxed – he's not relaxed...
"If you don't perform every week, you are going to hear a lot of things from club legends, from pundits, from the media, and sometimes they are right," added the Portuguese manager, speaking ahead of a league match at Tottenham Hotspur.
"To have the ability to understand that is normal and still maintain your level of confidence is really hard for a young kid, especially for a young kid that is a control freak, wants to control everything, and he's not going to control everything.
"He is going to be our striker for the long term. But he's going to have these struggles and these bumps during the ride, and that is a normal thing in football."
Following last week's 2-2 draw at Nottingham Forest, former United player and pundit Gary Neville said Sesko looked "awkward" on the ball, comparing him unfavourably to the club's other recent signings.
"Of course, nobody likes to hear that, but he (Sesko) struggled a little bit, and that is a fact. So, let's embrace that," Amorim continued.
"It's not personal (Neville's comments). That is what I try to explain to the players. That it is not personal. It's an opinion that is going to change in three weeks...
"My advice to Ben is you are going to get used (to it). And then it's going to be natural ... That is part of the process, and we are going to help him and we are going to protect Ben because he works really hard and we want to succeed."
United, eighth in the standings, face sixth-placed Tottenham later on Saturday.