All-Star point guard Fox has yet to play in the conference finals after injuring his right ankle in the second-round series win over the Minnesota Timberwolves.
Rookie Harper, who has stood in for Fox in the starting lineup, left Wednesday's game-two defeat with a thigh muscle injury.
The Spurs opened the best-of-seven series with a spectacular double-overtime victory in game one, with the Thunder bouncing back on their home floor in game two.
The Spurs will host games three and four on Friday and Sunday.
The winner of the best-of-seven series will advance to the NBA Finals.
The Thunder are also hurting. Swingman Jalen Williams, who has been hampered by a hamstring issue since the start of the playoffs exited after just seven minutes in game two and is questionable for Friday's game.