Italy international Scamacca was preferred to Antonio for Sunday's 1-0 win at Aston Villa and in the process made his first start in the Premier League since a £30million move from Sassuolo.
Asked how West Ham's all-time leading Premier League goalscorer will respond to the competition, Moyes replied: "It is great and what we want.
"I said to Mic a few weeks ago, there is no way we are changing. Mic has been so good. To try and do what Michy Antonio has done for us over the last two-and-a-half years, it would be very difficult to get somebody who could do that.
"What we have needed to do is get somebody, which we've known for a while, who we think can play one week or change as a substitute. We didn't have that option last year which we had today to bring Mic on with 30 minutes to go."
Summer signing Scamacca cut an isolated figure in the first half at Villa Park but West Ham improved after the break and the Italian tested Emi Martinez with one effort and was beginning to cause problems for the home back four before being brought off.
Moyes was pleased with the efforts of the 23-year-old, who has scored twice for the Hammers in the Europa Conference League this month.
"We probably gave Gianluca nothing in the first half, he had bits and scraps. We didn't give him much service and we didn't get any crosses into the box for him to try and get on the end of," the West Ham boss added.
"It just started before we took him off, we started to show more and he showed a couple of good signs.
"He is one of the main Italian strikers, he is young but we wouldn't expect him to hit the ground running exactly. I actually think it is harder for centre forwards than it is any position but we like him.
"What we see we like. He gives us a presence and has already scored us a couple of goals, which is a good sign for us."