The Spurs switched out the two big men while coach Gregg Popovich continues to try to beef up his thin frontcourt. San Antonio began the season with only Tim Duncan, Matt Bonner, Tiago Splitter and DeJuan Blair as options at center or forward.
Thomas is a 6-foot-9 rookie who played with Spurs swingman Kawhi Leonard at San Diego State last season. Leonard was drafted 15th overall and may get his first start against Houston on Wednesday.
Diogu played sparingly in two games after being signed Jan. 3.