Jim Harbaugh would rather show up to work in a Seahawks sweatshirt and Bermudas than provide a sliver of injury information about his players to the media, but he was, relatively speaking, Mr. Loose Lips on Friday afternoon.

Harbaugh divulged that left guard Mike Iupati wouldn’t play Sunday against the Broncos because he was still experiencing concussion symptoms, acknowledged that linebacker Patrick Willis was still in a walking boot because of a toe injury and was in the midst of discussing cornerback Jimmie Ward’s quadriceps injury before he slammed on the brakes: What in the world was he doing?

“It’s a lot of information,” he said. “I’m not used to giving that kind of information.”

No kidding. How are you feeling about this breach of personal policy, coach?

“I feel used, manipulated,” he said cheerfully.

Billy Hurst / Associated Press

Given his shorthanded team’s underdog status entering Sunday night’s visit to face the defending AFC champions, Harbaugh was in remarkably good spirits Friday afternoon. Or, perhaps, he was in remarkably good spirits because of the shorthanded 49ers’ underdog status?

The 49ers have been installed as seven-point underdogs against the Broncos, the most since they were 10-point underdogs at Philadelphia on Oct. 2, 2011. They won that game 24-23, and they are 4-0 when they are at least five-point underdogs in the Harbaugh era.

Perhaps it’s not a coincidence. Harbaugh, after all, has often said he finds being underestimated to be a “wonderful competitive advantage.” If so, the Broncos (4-1) are in danger against an opponent in full us-against-the-world mode.

Consider:

• The 49ers won’t have three All-Pro linebackers (Willis, Aldon Smith, NaVorro Bowman) and, perhaps, two of their top three cornerbacks (Tramaine Brock and Ward) against Peyton Manning, one of the best quarterbacks in NFL history. They also won’t have Iupati, an All-Pro offensive lineman.

• They’ll play at Sports Authority Field, where the mile-high elevation and the home team’s excellence explain the Broncos’ 17-2 record there since 2012.

• They are in the midst of a brutal six-game stretch that has them playing at Denver six days after a 31-17 Monday night win at St. Louis. Only six teams since 2010 have played a road game six days after a Monday night road game. Those teams are 2-4 in the second game of that back-to-back road stretch.

Given that travel grind, Harbaugh has eased up on this week’s schedule. The 49ers had an abbreviated practice Wednesday and were allowed to leave the facility early Thursday.

Still, tight end Vernon Davis said it’s an unforgiving back-to-back.

“That’s pretty brutal on the body,” he said.

Despite the curtailed schedule, Harbaugh said Friday that his team’s practice and preparation have been excellent this week.

“The focus is there, and a lot of that has to do with the Broncos,” Harbaugh said. “Such a good team. That tends to hone your focus as well.”

And so does being underestimated.

Eric Branch is a San Francisco Chronicle staff writer. E-mail: ebranch@sfchronicle.com Twitter: @Eric_Branch

Dominant dogs

The 49ers are seven-point underdogs against the Broncos, which marks the fifth time they’ve been at least five-point underdogs in the Jim Harbaugh era. They are 4-0 in those other games:

Spread: Eagles favored by 10

Result: 49ers 24, Eagles 23, Oct. 2, 2011

Spread: Lions favored by 5

Result: 49ers 25, Lions 19, Oct. 16, 2011

Spread: Packers favored by 51/2

Result: 49ers 30, Packers 22, Sept. 9, 2012

Spread: Patriots favored by 5

Result: 49ers 41, Patriots 34, Dec. 9, 2012