Motiejunas, 28, last played in the NBA at 20 17 for the New Orleans Pelicans. He has spent the last two seasons playing China.
San Antonio (42-31) has lost two straight after winning eight in a row and it is currently eighth in the Western Conference standings.
Motiejunas averaged 7.4 points and 3.8 rebounds per game in five seasons at the NBA, the first four of that came with the Houston Rockets.
The Spurs will play Boston on Sunday, the second game of a three-game roadtrip.