Ron Artest added 16 points and eight rebounds, Yao Ming and Rafer Alston scored 12 points apiece, and Luis Scola added 11 for Houston, which had all five starters in double-figures for just the second time this season. Alston also had six assists, and reserve Shane Battier scored 15 points.
The Rockets finished with 22 assists.
"We need ball movement," Alston said, "and the ball was hopping tonight."
McGrady, Houston's second-leading scorer, sat out for the second straight game because of a sore left knee. The team said McGrady also would miss Sunday's game in Denver and would meet with doctors on Monday to discuss treatment options.
Head, a soft-spoken shooting guard, had played in just five games and scored only 19 points all season before going 7-for-11 from the field Saturday night.
"It was just something that happened today," said Head, 2-for-5 from 3-point range. "My teammates found me all the time. I was making shots, penetrating to the basket and getting it in transition. It just worked today."
The Rockets shot 48 percent (39-of-81) and committed only seven turnovers to beat the Spurs for the fifth time in the last 16 meetings.
Head had become a virtually forgotten man on the Houston bench. Coach Rick Adelman had relied more on Aaron Brooks and Brent Barry to back up Houston's starting guards. Barry sat out Saturday with a strained right calf, making Head the next best option to fill in for McGrady.
"Luther had a chance to start and I can't say enough about what he did," Adelman said. "He exceeded everybody's expectations."
Not one to complain, Head has quietly gone about his business this season, waiting for his scant opportunities. He plays 1-on-1 games with rookie Joey Dorsey and reserve Von Wafer before every game to stay sharp.
"It is tough, I can't deny that," Head said. "That's what I've been dealt this year. I've just got to stay ready. Whenever my team calls upon me, I'm just going to come up there and try to play as hard as I can."
Tim Duncan shook off a bad start to score 17 points for the Spurs, who had held their last 11 opponents under 100 points. Tony Parker had 15 points and seven assists in his second game and first start since missing nine games with a sprained ankle.
"Houston came out and was more aggressive than we were," Spurs coach Gregg Popovich said. "They played a fine game for 48 minutes, as far as their aggressiveness and overall energy. We were inconsistent."
Houston shot 62.5 percent (15 of 24) in the first quarter to build a 33-20 lead.
The Rockets hit their first three shots of the second quarter and frustrated Popovich called a timeout. On the next play, Brooks promptly threw in a runner in the lane, got fouled by George Hill and sank the free throw to put Houston up 42-23.
The Spurs scored nine straight points late in the second quarter to cut Houston's lead to 50-40. San Antonio trailed only 56-49 at the break, despite Duncan going 1-for-9 from the field.
"I played an awful game in the first half," Duncan said. "I couldn't hit any of my shots."
Matt Bonner started the second half with a 3-pointer to cut San Antonio's deficit to four. The Rockets continued to move the ball and shoot well and finished the third quarter with a 15-6 run to take an 84-70 lead.
By then, the Rockets were recalling their Nov. 14 game in San Antonio, when they wasted a 14-point lead in the final 8 minutes and lost 77-75. Or their loss to Indiana on Wednesday, when they blew a 10-point lead in the fourth quarter.
"That's what Ron [Artest] and I talked about," Alston said. "When we get the lead, we've got to keep the lead and not just stop doing what we're doing. We're a team that gets a 10- or 15-point lead and then takes our foot off the gas pedal."
Head made sure that didn't happen this time. He opened the final quarter by stealing the ball from Ginobili, racing for a layup and converting a three-point play. He drove for another layup a minute later to put Houston up 89-72.
The lead swelled to 20 after Kurt Thomas lost the ball to Brooks, who sprinted to the other end for another layup with 7:03 remaining.
San Antonio shot 38 percent (29 of 77) and went 5-for-17 from 3-point range.
The Spurs outrebounded Houston 39-34. ... Bonner scored 17 points for the Spurs. ... The Rockets have won the last three games against the Spurs in Houston.