SANTA CLARA, Calif. -- San Francisco 49ers rookie quarterback Trey Lance was thrown into last week's game at halftime on short notice. This week, there's no such uncertainty.

Lance will make his first NFL start Sunday against the 4-0 Arizona Cardinals, Niners coach Kyle Shanahan said Friday. That announcement comes after a week in which Lance took the reps with the starting offense as starter Jimmy Garoppolo has dealt with a strained and bruised right calf.

Garoppolo is listed as out on Friday's injury report after he did not practice all week. The 49ers could also be without tight end George Kittle, who is listed as doubtful with a calf injury of his own.

Which means Lance, the No. 3 overall pick in this year's NFL draft, could be starting without the Niners' star tight end available and veteran Nate Sudfeld serving as Lance's backup after he's elevated from the practice squad.

"I'm excited but I'm not going to make it bigger than what it is," Lance said. "I was hoping Jimmy would be healthy, but he's done an awesome job and him and Nate have helped me out a ton this week and (quarterbacks coach) Rich (Scangarello) and Coach Shanahan as well. So, I'm excited for the opportunity."

Lance, who will be 21 years and 154 days old on Sunday, will become the fourth-youngest quarterback at the time of his first NFL start. He will also attempt to become only the second quarterback (joining Phil Simms in 1979) selected in the first round to win his first start against a team that's 4-0 or better.

While this will be Lance's first start, he is coming off his first extended playing time. He replaced Garoppolo at halftime of last week's loss to the Seattle Seahawks, learning he was going into the game just moments before the third quarter began. After some shaky early moments, he settled down a bit in a performance Shanahan said had "some good and some bad."

Lance finished that game 9-of-18 for 157 yards with two touchdowns and ran seven times for 41 yards. Having the full week of practice with the starters not only allowed Lance to get more comfortable but also gave the 49ers the chance to build out a game plan with more Lance-specific plays in it.

Still, it's hard for anyone to predict what Lance's first start will look like.

"I'm excited to see him play," Shanahan said. "I know he's got the ability to do a lot of things but it's going to be his first time out there for the whole game. I'm just pumped to see him play but I don't go into it expecting anything. You try to prepare a guy as good as you can from our standpoint. I know he's done his part preparing and when Sunday comes that's usually the spot where you get to have fun and cut it loose and you live with the results."

On Wednesday, Garoppolo was hopeful he'd be able to play against the Cardinals but acknowledged that he'd be "battling through" the calf injury rather than being fully healthy.

That was an important distinction given that Garoppolo attempted to return too quickly from a high ankle sprain in last year's Week 5 loss to the Miami Dolphins. A clearly hobbled Garoppolo went 7-of-17 for 77 yards with no touchdowns and two interceptions before heading to the bench in that 43-17 loss.

As late as Friday morning, the Niners and Garoppolo had held out hope he would participate in Friday's practice in some capacity. But after some pre-practice testing, it was clear Garoppolo wouldn't be able to give it a shot.

"Somebody has got to protect them from themselves," Shanahan said. "(He) tried to get warmed up and get ready to go. It wasn't reacting the right way so we've got to make the smarter decision."

Garoppolo will have additional recovery time for this injury, though, with the Niners heading toward a Week 6 bye. The hope is that he should be able to be healthy for the Oct. 24 game against the Indianapolis Colts.

Garoppolo said Wednesday that while he was hoping to play this week, he was also spending time helping Lance prepare when possible.

"It's kind of the balancing act," Garoppolo said. "If Trey has any questions, anything that just I've ran in the past or that he hasn't ran, (I'm) helping him out any way with that."