Mutombo, who supplants 39-year-old Sam Cassell as the league's oldest active player, appeared in 39 games for Houston last season, logging most of his minutes after Yao broke his left foot in February. Mutombo averaged five rebounds, three points and 1.2 blocks.
He became a free agent after last season and contemplated retirement before deciding to come back for an 18th season.
The Rockets dropped to 20-12 on Monday after an embarrassing 89-87 loss to Washington. Nearly every player on the Houston roster has missed games due to injury this season and Yao has sat out at least 25 games because of injuries over the past three seasons.
Mutombo started 25 games last season, mostly after Yao broke his foot, and averaged 16 minutes per game. The Rockets traded Steve Francis to Memphis on Christmas Eve, creating salary space to add Mutombo, the NBA's second career shot blocker behind Hakeem Olajuwon.
The Rockets play Milwaukee on Wednesday and Mutombo is expected to join the team, though it's not known when he'll start playing again.
The 7-foot-2 Mutombo has played for six NBA teams -- Denver, Atlanta, Philadelphia, New Jersey, New York and Houston. He was traded from the Knicks to the Rockets on Sept. 8, 2004, for Adrian Griffin, Eric Piatkowski and Mike Wilks and averaged four points, 5.3 rebounds and 1.3 blocks in the 2004-05 season.
Mutombo's teams have made the playoffs in 13 of his 17 seasons in the league, including three of his previous four with the Rockets. He averaged 14 points and 14 rebounds to help the Sixers reach the NBA finals in 2001.
The Rockets have lost six straight playoff series and haven't advanced to the second round since 1997.