Stack, a 6 ft 11 in (211 cm) tall forward/center, from the University of South Carolina, was selected by the Cleveland Cavaliers, in the second round, with the 48th overall pick of the 1998 NBA draft.

He saw limited playing time in two NBA seasons with the Cavaliers, as the backup to the team's All-Star center Žydrūnas Ilgauskas.

Stack was then signed by the Greek EuroLeague club Olympiacos Piraeus, in August 2006, and at that time, he was reported to have been the highest paid center in Europe.

While playing with the Greek Basket League club Aris Thessaloniki, Stack adopted the citizenship of North Macedonia, under the name of Ruan Stik.

[1] Because of that, he was able to play as a European, with a Bosman passport, and thus not count as one of the two non-Europeans that were allowed per team in the Greek Basket League at that time.