After hinting at the move since Saturday's 16-13 loss to Auburn, Spurrier made it official following Monday's practice.
"As most everyone knows our quarterback play has not been as good as we hoped and we've got to do something," he said.
The Gamecocks (4-1, 2-1 Southeastern Conference) seemed set at the position with Garcia, a fifth-year starter who had a breakout year in 2010 as he passed for 3,059 yards and 20 touchdowns in helping the Gamecocks reach their first SEC championship game.
But Garcia hasn't come close to that production this fall. His completion percentage has dropped from 64 percent last year to 51 percent this season. He's thrown nine interceptions and just four touchdowns in South Carolina's first five games.
Garcia was 9 of 23 for 160 yards and two interceptions against an Auburn pass defense that came in ranked last in the SEC.
"It wasn't that hard a call," Spurrier said. "At some point, we've got to do something to shake up this offense and this is the first step."
Garcia has been suspended five times in his college career, including twice this spring.
Spurrier hoped it wasn't too late to spark an offense that was expected to do so much more than it has done this year.
The Gamecocks feature the SEC's leading rusher in Marcus Lattimore and another all-SEC performer in receiver Alshon Jeffery. The offensive line has as much experience as any in Spurrier's seven seasons, with three seniors and a junior. Garcia figured to be the steady hand under center making it all run smoothly.
Instead, Garcia has looked as lost as he's ever been in five troubled seasons under Spurrier.
The coach said Garcia took the news well. "I think he anticipated it," Spurrier said.
Garcia was not available to the media after practice.
It's not the first time this season Spurrier has given Shaw a chance to start. Spurrier ended Garcia's streak of 28 straight starts by opening with Shaw in South Carolina's first game of the season with East Carolina.
Shaw and the Gamecocks, though, struggled to make plays and, trailing 17-0, Spurrier sent in Garcia. The senior showed his experience in rallying South Carolina to a 56-37 victory.
Spurrier said after beating the Pirates that Garcia was South Carolina's quarterback. That changed with a month of spotty performances that relied on Lattimore's powerful runs rather than a solid passing game.
Spurrier realized something had to change after the Auburn loss when the Gamecocks had a season-low 289 yards of offense and that led to Shaw.
"This is his game. It's his opportunity to go play and we think he's ready," Spurrier said. "We're going to find out Saturday."
Shaw has seen limited action since East Carolina. He thinks he's prepared for another chance. "It's an opportunity I'm willing to take advantage of. I think I'm ready to play," he said.
This time, Shaw won't have Spurrier's quick quarterback hook hanging over him. Spurrier said unless Shaw got hurt, "this is his game."
Shaw will get a strong test from Kentucky (2-3, 0-2), which is fourth in SEC pass defense at 163 yards allowed a game.
It has not been the easiest year for Garcia. He was suspended a final time in April for "unacceptable behavior" at a life-skills seminar and was kept from all team activity. Garcia met a list of undisclosed conditions set by the school and he was reinstated for summer practice. Spurrier told him he would have a full-blown competition for his starting job -- one he apparently lost when Spurrier gave Shaw the nod for East Carolina.
This past week, Garcia's grandfather died, although he remained with the team to play against Auburn.
Garcia didn't know why the offense wasn't clicking when asked about it after the loss. "It just didn't work out for us," he said.
Spurrier said Garcia, who has made 34 career starts at South Carolina, would be Shaw's backup.