The pair will fight for the WBO belt at Vector Arena in less than a fortnight, on December 10.
Parker's trainer Kevin Barry said the 24-year-old heavyweight's past four years as a professional - during which he has won all 21 of his fights - has served as an apprenticeship.
Ruiz has spent eight years as a professional and is unbeaten in 29 fights.
Barry said that gives the visitor a distinct advantage.