Italy international Scamacca joined West Ham last month on a five-year contract but Moyes said Sunday's match comes too soon for the 23-year-old.
"Scamacca has only just really joined us, so he's probably a little bit behind, so I'm not expecting him to be available this weekend," Moyes told reporters.
"He's not had the chance to play... so it probably makes it very difficult for us to put him out on the pitch so soon. His training is a little bit behind compared to the others. He has trained, but maybe not as regular and as consistent.
"The transfer took quite a bit off as they pulled him out of some games and training at the time, so we've got him in and I like what I see, but we're trying to get him in the best condition we can and get him ready to play."
Moyes, whose side finished seventh last season, will also be without another Italian in Angelo Ogbonna (34) for up to four weeks as the defender builds up his fitness following a knee injury.