SAN FRANCISCO — Comparing the 49ers with Trey Lance at quarterback and the 49ers with Jimmy Garoppolo at quarterback is like comparing apples to armadillos.
No other NFL offense has such a wild style disparity between the starting quarterback and the backup. So this is no easy quarterback dilemma the 49ers face.
When the 49ers traded a ton of future talent to draft Lance, head coach Kyle Shanahan envisioned — at least hoped for — a smooth quarterback transition at some point.
Garoppolo, it would seem, will get his starting job back when the team returns Oct. 24 after its bye week, assuming he has recovered from the calf injury that kept him out of Sunday's 17-10 loss at Arizona.
Several media outlets also reported Monday that Lance suffered a sprained knee against Arizona and may not be ready to play against Indianapolis in two weeks. This is the 49ers' bye week.
But even before news of Lance's knee injury, Shanahan was pounding that theme with greater intensity. In a pregame TV interview Sunday, he emphasized that this is Garoppolo's team.
To which many fans, and maybe some 49ers players, say, "Why?"
Why should it be so clear-cut? What has Garoppolo done to make his status so secure?
You can't blame Shanahan. Lance is as green as guacamole. On the 49ers' opening possession Sunday, near midfield, Lance sailed a pass 10 feet over the head of a wide-open receiver. The ball was intercepted.
It was the type of pass that proves a rookie isn't quite ready. It's also the type of pass that Garoppolo has been known to throw.
It was also the last what-the-hell? pass of the day for Lance.
As he has done before, Lance took a deep breath and got back to business. He made some nice throws, some smart decisions. On 3rd-and-3 at the Cardinals' 32 in the second quarter, Lance threw a frozen rope to Brandon Aiyuk at the sideline, into a window the size of a matchbook.
Later in the half, Lance threw what would have been a 40-yard completion had Mohamed Sanu not dropped the ball.
In order to be proclaimed the starting quarterback for the 49ers' next game, Lance would have needed to knock the socks off his head coach, and wind up with a win against the Cardinals (5-0). No socks were knocked off, but many eyes were opened.
Lance did not do a Colin Kaepernick, who made his big debut as 49ers starter in 2012 with a sizzling performance in a "Monday Night Football" win over the Bears.
Lance was 15-for-29 passing for 192 yards and a 58.4 rating. If Garoppolo put up passing stats like that, he would be roasted. But Lance also ran 16 times for 89 yards.
It would be a stretch right now to say that Lance is a better NFL quarterback than Garoppolo. But Lance is a very different quarterback, quarterbacking a much more exciting offense.
Style-wise, we're talking about tortoise and hare. Shanahan, to his credit, recognized that and called a completely different game than he calls for Garoppolo. The coach turned his young quarterback loose, and also gave him drop-back plays.
On the 49ers' first possession of the second half, they faced 4th-and-5 at the Arizona 40. Punt, right? No, Shanahan said "Go."
In the radio broadcast booth, 49ers color commentator Tim Ryan said, "That's what Trey Lance does to a play-caller!"
The bad news on that play: Lance dropped back to pass and didn't look away from his target, Sanu. Rookie mistake.
The good news: Lance lasered a 6-yarder to Sanu for the first down.
The Trey Lance bandwagon was taking on passengers like a Titanic lifeboat. But he and his teammates couldn't close the deal, as the drive was stopped.
Lance did emerge as a genuine double-threat quarterback. On designed runs and on broken plays, he flashed NFL-level running skill, with a style all his own. Where Kaepernick, for instance, got his big runs by outsprinting would-be tacklers around the corners, Lance — based on Sunday's performance — is more of a tough-guy inside runner, looking to pile-drive forward rather than lean back into a slide.
Lance breaks tackles. He runs to contact.
Yes, that's dangerous. Reportedly, NFL players get hurt.
Lance even seemed to inspire his teammates. Fullback Kyle Juszczyk took a swing pass from Lance and hurdled a would-be tackler. Fullbacks don't do that.
And here's a bonus: Lance made several passes from the pocket, looking quite comfortable.
One thing Lance does: He changes the big picture every few minutes. Late in the third quarter, the 49ers were still scoreless, which is no way to promote the future of a rookie starting quarterback. Then Lance led a wild 59-yard touchdown drive that included approximately 50 holding calls on the 49ers' linemen.
The drive included three Lance runs. He lost his helmet on one run, the second lost helmet of the day. His head might be screwed on right, but his headgear is not.
Lance brings thrills. But thrills are not always good. This is NFL football, not a James Bond movie. And yet, Shanahan has to recognize that Lance, unlike Garoppolo, scares defenses, takes them out of their groove.
On 3rd-and-7 early in the fourth quarter, Lance scrambled for 15 yards.
"Trey Lance is a huge Band-Aid when everything breaks down!" Ryan gushed.
But how often does that happen in an NFL game?
Scott Ostler — Email: firstname.lastname@example.org Twitter: @scottostler — is a columnist for The San Francisco Chronicle — www.sfchronicle.com — which is distributed by Tribune Content Agency, LLC.