WBA, IBF and WBO champion Joshua was originally set to face Jarrell Miller in a heavyweight showdown in New York City on June 1, but that bout was scrapped after Miller failed drugs tests.
Ruiz stepped up to take Miller's place, with the American-born 29-year-old – who is of Mexican heritage – having won 32 of his 33 professional fights.
On Wednesday, Joshua urged current WBC champion Wilder to clear the path for an undisputed heavyweight title fight, should the Briton beat Ruiz.