Williams was ruled unable to play in the huge Saturday night clash, with a spot in the NBA Finals at stake, due to a left hamstring strain, according to the injury report.
The Thunder star wing has been hampered by a hamstring issue since the start of the playoffs.
He exited after just seven minutes in game two of the Western Conference finals series against the Spurs, and missed games three, four, and five.
Williams managed just 10 minutes in Thursday's emphatic 118-91 loss, and did not feature in the fourth quarter.
Defeat to San Antonio tied the best-of-seven series at 3-3, with the final game taking place in Oklahoma City on Saturday.
Williams had played a vital role in the Thunder's successful run to the NBA championship last year.
The victors on Saturday will play the New York Knicks for this year's title.
The Knicks are in superb form and well rested, after sweeping both the Cleveland Cavaliers and Philadelphia 76ers in an 11-game winning streak on their way to the finals.