He did not start and played 26 minutes, totaling nine points, two rebounds, two assists and one block.
Williams made 4-of-6 field goal attempts and converted 1-of-2 3-point attempts for Boston, which led, 83-75, at the end of the third quarter.
Miami leads, 2-0, in the best-of-7 series, claiming the first two games on the road.
Game No. 3 is slated for Sunday. Tipoff is scheduled for 8:30 p.m. EDT at Kaseya Center in Miami, Florida. TNT will televise the contest.