In Sunday's preseason finale against the Las Vegas Raiders, the Niners offered a glimpse of how that might work. For the only two drives that most of San Francisco's starters were on the field, Shanahan rotated his two signal-callers liberally.
Of a possible 24 plays, Garoppolo took 14 snaps and Lance took 10. On the opening drive, it was an even split on the 10-play drive, with Garoppolo handling nine plays to Lance's five on the second. Both drives resulted in rushing touchdowns, another thing the quarterbacks split with one for each of them in San Francisco's 34-10 victory.
"I think coming into it, it was weird at first, just having to process the whole thing," Garoppolo said. "But after being out on the field, it was fun. When we're clicking like that and the defense is scrambling like that, it makes it hard. We're trying to win games out there, so whatever it takes, we're going to do that."
Garoppolo finished 4-of-7 for 64 yards for a passer rating of 87.8. He also had two carries for 3 yards and a touchdown on his 14 total snaps. Lance was 6-of-13 for 46 yards for a passer rating of 55.3 and ran six times for 8 yards and a score while playing 33 total snaps. Nate Sudfeld replaced Lance on the Niners' second drive of the third quarter.
Last week, Shanahan acknowledged that he wasn't quite sure what form a potential quarterback platoon would take or when it would be deployed in regular-season games. Shanahan has likened the idea to how the New Orleans Saints used Drew Brees and Taysom Hill in recent years, though Lance won't line up at spots other than quarterback as Hill so often did.
After Shanahan made those comments Wednesday, the Niners began rotating the two quarterbacks a bit during "move the ball" periods in that practice. In Thursday's session, the final practice of the preseason open to media, Lance and Garoppolo rotated but only during red zone periods.
On Sunday, Shanahan said he planned to rotate the quarterbacks, but he allowed the implementation of it to happen organically.
"I thought they did a good job," Shanahan said. "By no means was it perfect but I thought the function of it and going in and out and not knowing who it was going to be, it wasn't a planned rotation. We tried to make it situational."
Part of Shanahan's motivation for subbing his quarterbacks so often was to get them and the offense used to how it would work in a game situation since the play clock runs the same regardless of who is at quarterback and coach/quarterback communication cuts off at 15 seconds.
On the first series, Lance entered for Garoppolo twice to cover his five snaps and he entered four times for his five total snaps on the second drive. For each time he entered, Lance stayed near Shanahan on the sideline, waited to hear his personnel grouping called and then entered.
Once he got to the huddle, Lance would get the playcall and deliver it to his teammates. Like Garoppolo, Lance said he enjoyed the rotation.
"It's fun," Lance said. "I trust Coach Shanahan and I know Jimmy does and our whole team and locker room does. He's going to find ways to get the guys that need the ball, the ball, and do whatever it takes to win games ... It was good to get the rhythm."
While Lance and Garoppolo made the most of their platoon, they also both seemed to narrowly escape injury.
Garoppolo dived for the game's opening touchdown as two Raiders converged on him. He was OK, but it was a questionable decision in a preseason game.
"If I would have gotten hurt, that would have been a terrible idea," Garoppolo said. "But yeah, knock on wood, thankfully good things came out of it."
Later in the first half, Lance jammed his finger on the helmet of a Raiders defender after making a throw. He was seen holding the finger for a few plays, but he finished out his portion of the game without incident.
"We'll see how it feels tomorrow when we can tell a little bit more, but not too worried about it at all," Lance said.
After the game, Shanahan again declined to name a starter for the Sept. 12 opener against the Detroit Lions, repeating that he has a "pretty good idea" of who it will be. Most signs still point to that being Garoppolo with Lance getting plenty of opportunities to work in where Shanahan sees fit.
Which means it's best for the Niners' offense to get used to seeing either quarterback at any given time.
"It catches you off guard because I'm used to having Jimmy in there the entire time, so then when Trey comes in, it's just like 'Oh, here comes Trey, what are you doing in here, 5?'" running back Raheem Mostert said. "But it's cool. It's definitely unique and it's honestly fun."