The 29-year-old joined the Black Cats from Juventus last summer, but struggled to find his best form for the club as they spent most of the season battling against relegation.
However, Poyet has been impressed by how he has performed in pre-season and believes that he will enjoy a successful campaign at the Stadium of Light.
"Giacchi is one of our biggest signings this summer and I am being serious," The Guardian quotes Poyet as saying. "I know where I can play him, how he fits in.
"He's just played two 90 minutes for Italy and I have changed my mind completely about him. Giacchi has made me happy. I think he's going to be great for us. I see him as a new player, a totally new player."
Giaccherini has won 21 caps for the Italian national side.