Mutombo, 42, an 8-time NBA All-Star and 4-time Defensive Player of the Year, was carried away on a stretcher in the first quarter of the Rockets' 103-107 loss to the Portland Trail Blazers.
Last year when Yao Ming broke his leg, Mutombo picked his venerable body up off the bench and helped the Rockets hold their season together with a 20-12 record.
Last month, when Yao went down again with a stress fracture in his left foot, Mutombo, 41, stepped up once more, and the Rockets have remained in the hot playoff race with 12 wins in 15 games.