Bigger key is how Broncos' QB Joe Flacco plays against Fangio's former Bears defense.

ENGLEWOOD, Colo. — Vic Fangio knows so much about the Bears, he’s not going to share.

Bears head coach Matt Nagy knows so much about Fangio, he’s not going to share.

“Yeah, we’re both sandbagging right now,’’ Nagy said in a conference call Monday with the Denver media.

Fangio’s Broncos play his former Bears this Sunday in the first game since Denver’s football stadium has been renamed Empower Field at Mile High.

There is inside knowledge and there is gamesmanship. Bears quarterback Mitch Trubisky, for instance. Anyone who saw Trubisky play against the Green Bay Packers in the Bears’ 10-3 season-opening loss at Chicago’s Soldier Field last week knows he’s a terrific athlete who can run, but his passing accuracy is suspect from the pocket.

But that’s not how Fangio described Trubisky on Wednesday. The Bears defensive coordinator-turned-Broncos head coach sees a complete quarterback in Trubisky.

“He is dangerous when he is scrambling,’’ Fangio said at his press conference Wednesday. “He is a very good athlete, he is fast and he is elusive. He has a good feel for when to pull it down and run. He’s made a lot of plays in his short career doing that.

“Yet, he’s a very accomplished passer when he’s inside the pocket. Can make all the throws they ask him to make and he’s a damn good player.’’

What everyone knows of the Bears’ defense that Fangio left behind for Chicago’s new defensive coordinator Chuck Pagano is Broncos’ quarterback Joe Flacco will not be able to hold on to the ball very long.

The Bears bring the pass rush, led by Khalil Mack and Leonard Floyd. If only throwing it quick were that easy.

“We understand that these guys are a really good defense and have been playing well for a pretty good amount of time now and have a good front,’’ Flacco said. “(But) there’s no way I can just drop back and say, ‘I’m going to get rid of the ball faster.’

“First of all, a guy has got to be open, and second of all we’re not just going to change—you don’t just automatically blink and change everything to three-step drop three-yard routes. That’s not going to win you the football game anyway.”

The Broncos better block then. And Mack is the priority. Fangio, who was in charge of the Bears’ defense from 2015-18, explained that he didn’t know much about Mack when Chicago acquired him from Oakland right before the season opener last year.

“And then when we got him and it was just after a few days, it was like, ‘Holy (bleep),’’ Fangio said.

The Bears must win Sunday not because they can’t lose to Fangio, but because they lost to Green Bay last week. It’s a must win for Fangio’s Broncos, too, but only because they lost to the Raiders on Monday night. The prospects of 0-2 make Sunday a must win for both.

“It really is just another game, especially coming off of our last game,’’ Fangio said. “We just need to go out there and play a little bit better no matter who we’re playing. It’ll be a little different once I see all those guys in pregame and whatnot. But after they kick it off, it’ll be another NFL football game.”