Reports indicate Leverkusen forked out around €22m (R340m) for the 19-year-old.
Retsos, who has represented Greece at youth level, will don jersey number three at BayArena.
Reports indicate Leverkusen forked out around €22m (R340m) for the 19-year-old.
Retsos, who has represented Greece at youth level, will don jersey number three at BayArena.