Oct. 21, 2021, is Denver’s do-or-done date.

If the Broncos bust in Cleveland, they will fall below .500 for the first time this year, likely end with a losing record for the fifth season in a row (which last occurred in 1968-72), probably suffer the humiliation of not reaching the postseason in the only six-year span since 1971-1976, drop at least four straight for the 19th season (including 2017, 2018 and 2019) and be alone in last place this weekend in the AFC West, where they wound up twice in the past four seasons.

Big game, huh?

The Browns are hurting bad. They definitely will play without quarterback Baker Mayfield, who started 51 consecutive games, and running backs Nick Chubb and Kareem Hunt. Wide receivers Odell Beckham Jr. and Jarvis Landry are up in the air, but neither is at full-go, and starting tackles Jack Conklin and Jedrick Willis Jr. are injured, but will try. And 13 other Browns are not feeling so good themselves.

The Cardinals hammered the Browns in Cleveland Sunday 37-14. Seemed just like the Broncos’ beatdown by the Raiders.

Case Keenum, who was 6-10 as the Broncos’ quarterback for all 16 games in 2018, hasn’t started since he was with the Washington Football Team two years ago. We’ve seen his act. Thank you, football deities.

This should be an imperturbable situation for the Broncos. Except.

They played Sunday against a team supposed to be like chickens I watched as a kid on my grandmother’s farm that ran amok when their heads were chopped off before dinner.

Cleveland is in its own chaotic, confused circumstance.

But these aren’t the Broncos of 1977, 1987 or 1997. They are more 2017 (when the record also was 3-3).

Quarterback Teddy Bridgewater played Sunday like Keenum of three years ago. He’s been limping around Dove Valley with a delicate left ankle and quad. Although listed as questionable for Thursday night, Bridgewater will start. The question is: Will he close?

It’s feasible that two former Broncos’ starting quarterbacks – Keenum and Drew Lock, not Kyle Orton and Craig Morton – could participate.

Is a 75 percent Teddy superior to a 100 percent healthy Drew? Will a 100 percent Keenum be an improvement over a 45 percent Mayfield?

The Browns certainly believe Keenum is the correct choice. Obviously, Vic Fangio and Pat Shurmur still prefer Bridgewater. Lock was thisclose to outdueling Bridgewater in the training camp-exhibition “Decision’’, but Lock no longer is a viable starting alternative to the Broncos’ brain trust when Bridgewater is out for almost a week with a concussion or barely can climb one step to the press podium at the practice field. Teddy didn’t even discuss his hobble.

The Broncos have a serious dilemma at linebacker, too. Six inside and outside guys are out. Bradley Chubb still is recovering from ankle surgery; Josey Jewell and now Alexander Johnson are gone for the season with torn pec muscles; Andre Mintze (hamstring), Baron Browning (concussion) and Aaron Patrick (ankle) won’t play. Jonas Griffin (hamstring) and Natrez Patrick (leg) already were on injured reserve.

Inside linebackers Curtis Robinson (an undrafted free agent rookie) and Barrington Wade (claimed on waivers from the Ravens) have been activated from the practice squad for the game. Micah Kiser, signed off the Rams practice squad a month ago, will start.

The defense might consider a 7-2 instead of 3-4.

The Broncos are 40 years removed from Orange Crushers Randy Gradishar, Tom Jackson, Bob Swenson and Joe Rizzo.

So, Von Miller, who has promised to destroy whatever tackles the Browns start, and Malik Reed must carry the burden for the Broncos against Keenum and whatever running backs.

Thursday Night Football won’t be Must View TV in America, given that the Dodgers and the Braves play, and NBC offers various versions of Law & Order.

Flaw & Disorder could be theme for the Not Ready for Prime Time players in Cleveland. Both teams seek to avoid sliding to 3-4, with the Browns next playing the Steelers, then the Bengals, and the Broncos at home against WFT and a road game with the Cowboys.

Four successive setbacks would scorch the seats of Fangio, Shurmur, Bridgewater, George Paton, Joe Ellis and even John Elway, who has distanced himself.

Because the Broncos would be done.