Greece international winger Charis Mavrias has arrived on Wearside for a medical ahead of a £2.5m move from Panathinaikos.
The 19-year-old, who has been capped twice by his country, has begun talks at the Stadium of Light over personal terms.
Mavrias has made more than 60 appearances for Panathinaikos, scoring six goals, and has Champions League experience.
Three years ago he made his debut in Europe's elite club tournament against Rubin Kazan at the age of 16 - becoming the second-youngest debutant in that competition behind Celestine Babayaro.
Paolo Di Canio has already received considerable backing from owner Ellis Short this summer, with 10 fresh faces drafted into the Stadium of Light.