DENVER (AP) — If the Broncos guessed right, Paxton Lynch will be a very good quarterback someday.

Sunday he looked like a rookie, though, and Denver walked away with its first loss of the season along with the sobering realization that the quarterback they need now is Trevor Siemian.

Siemian sat out of Denver’s 23-16 loss to Atlanta with a sore left shoulder he received after being hurled to the ground last week against Tampa Bay. Against Atlanta, Lynch made his first NFL start and went 23 for 35 for 223 yards, but also took six sacks. He looked every bit like an indecisive rookie and not much like he did in his relief appearance last week against the Bucs.

“Obviously, you have some butterflies,” Lynch said. “It’s my first NFL start, it’s not a preseason game and your team’s 4-0.”

Given all that, not even a week to prepare for a team that turned out to be much better than the Bucs was enough. Lynch looked like the rookie who couldn’t come close to winning the job in the offseason, after John Elway traded up five places to pick him in the 26th spot.

“He had a really good week of practice,” coach Gary Kubiak said. “We felt really good about him going into the game.”

The way Denver’s defense had been playing, why not?

By sweeping their first four games with Siemian as the starter, the Broncos dismissed all those who doubted a repeat was possible with an offense that had to reinvent itself following the departure of Peyton Manning and Brock Osweiler.

It’s a delicate balance, and the defense helped validate Kubiak’s decision to start Siemian, the seventh-round pick out of Northwestern in 2015, over Lynch and Mark Sanchez, who didn’t even make the team after being acquired in the offseason.

Turned out, Siemian wasn’t simply holding down the spot. He turned out to be quite good, with six touchdown passes and a 99.6 passer rating.

One debate that gained steam in Denver last week — now almost laughable — was what to do with this abundance of riches. Lynch threw for 170 yards and a TD last week and helped the Broncos pull away from Tampa Bay. How long could Siemian possibly remain a Bronco, given the rookie’s all-but-inevitable elevation to the starting position?

Save that one for later.

Kubiak said Siemian practiced all week and was “very close” to getting the start for this game. “We just didn’t think he was ready,” the coach said.

Unsaid in all that is that Denver’s next game is Thursday at San Diego, and by sitting out against the Falcons, the hope is that Siemian might be healthy enough to start again.

After the Atlanta game, that plan takes on a greater sense of urgency.

Indecisive in picking out receivers, Lynch held the ball too long and took six sacks, along with eight more hits.

But this wasn’t all his fault.

For Siemian’s four starts, the defense created eight turnovers, many of which either went for touchdowns or put the offense in good field position. The defense kept Denver in the lead or within a score for the majority of the first quarter of the season.

For Lynch’s first start, the defense promptly gave up a 75-yard touchdown drive and the Broncos trailed 7-0 less than 4 minutes into the game.

“They’re a team that plays a lot of ... zone, and you have to be very patient,” Kubiak said. “You get down 13 to them and you can’t just pick away to get yourself back in the game. You start pulling down and getting it downfield too much, it’s tough.”

Lynch doesn’t know when his next start will come. One thing seemed clear after this one: Denver’s quarterback of the future is just that. When healthy again, Siemian will be the starter.

“I’m upset, obviously, because we lost, but I’m happy I got the first one under my belt,” Lynch said. “If we get put in this situation again, I’ll play better than I did the first time.”