ENGLEWOOD, Colo. — Peyton Manning versus his old team in a do-or-die situation. With that storyline set to overshadow everything else revolving around the Denver Broncos and Indianapolis Colts this week, it seems like this divisional matchup was predestined.

But plenty of other factors will help decide whether the Broncos move on to the AFC Championship Game or see their season end in the divisional round for the third time in the last four seasons: how well each team’s improving ground game fares, which team can avoid turnovers, and, finally, how each defense handles an attack that has more trustworthy options than each did in their Week 1 duel.

The Broncos held off a second-half surge from the Colts to win 31-24 in the regular-season opener, but both teams are different now. The Broncos changed their offensive line and their overall emphasis, while their overhauled defense eventually found cohesion and rose to a top-three unit.

The Colts shuffled their line, developed pass-rushing threats to replace the injured Robert Mathis, and eventually found a productive running back in Daniel “Boom” Herron to help take some pressure off quarterback Andrew Luck. But both teams remain capable of shredding a defense through the air if the need arises.

“When you look at that big of a gap, their (offensive line) has changed, our O-line’s changed. It was a tough opener, they’re a very, very good football team,” said Broncos coach John Fox. “(Luck) is as good a young quarterback as there is in the league. Much of that has not changed.”

But Luck can now spread the football to rookie wide receiver Donte Moncrief, who grabbed a 36-yard touchdown pass in the Colts’ wild-card win Sunday, and tight ends Coby Fleener and Dwayne Allen, each of whom grabbed eight touchdown passes in 2014.

That makes the Colts even more dangerous than in Week 1, when they fell behind 24-0 but nearly came all the way back. It took a fourth-and-goal stand in the third quarter and a late fourth-down deflection from rookie Bradley Roby in the final two minutes of regulation to prevent what would have been a colossal collapse.

But the Broncos’ defense is different, as well. In Week 1, it boasted a starting lineup that included nine players who did not play in Super Bowl XLVIII seven months earlier. Cohesion took a while to develop, and the Broncos’ defense surrendered an average of 23.0 points in the first 11 games — but allowed just 14.0 per game in Weeks 13-17. They’ve also allowed fewer touchdowns per game in the last five weeks — 1.8 a game, compared with 2.6 per game in the first 12 games.

“We’ve got that chemistry now and at that time, that was kind of our first time learning how to play with each other, and all new guys,” said cornerback Chris Harris. “So now, we’re playing them now, (and) we’re full strength.”

The multi-score leads that the Broncos nearly blew in their first two games were a distant memory by the end of the season — and that might be something that proves vital next week: the ability to finish off a foe, a trait the defense did not have when the season began.

“Something we’ve developed throughout the season was having that killer instinct to put teams away and not giving them any hope,” said defensive tackle Terrance Knighton. “If we’re up in the game, that’s something we’ll reiterate. And if we’re down in the game, it’ll also be something that we talk about as far as keep fighting and have that mentality to never stop playing.”

Luck and the Colts are much more than they were when the season began. The Broncos believe they are, as well.

The Broncos’ defensive improvement was built on cohesion and the maturation of Brandon Marshall, who began the season as a fill-in for Danny Trevathan at weak-side linebacker and ended it as an essential component whose absence directly led to the loss at Cincinnati.

Marshall suffered a mid-foot sprain in San Diego on Dec. 14 and did not play in either of the last two regular-season games. But he returned to practice Monday.

“It’s good to see him out there,” said Knighton. “He’s not quite himself yet, obviously. He didn’t take much of the team reps but we’ll be fine. It’s still a long week, a lot of days left and he’ll be ready by Sunday.”

Rookie Todd Davis replaced Marshall in the lineup and improved from his first to his second start, but given how the Chargers, Bengals and Raiders exposed the Broncos on underneath routes from their running backs in Marshall’s absence, the Broncos cannot expect to make a deep run without the third-year linebacker.

“Every series he’s making a play. Every. Series,” said safety Rahim Moore. “You don’t normally get that out of a linebacker that much.”