In the big rematch of last season’s neck-and-neck AFC Wild Card game, the new-look Denver Broncos take on a healthier and vengeful Pittsburgh Steelers team that is looking for blood after the punch in the gut that was a Tim Tebow to Demaryius Thomas strike that went 80 yards on the first play from scrimmage in overtime.

Let’s break down the major changes for both teams and what to look for in the marquee matchup in the year’s opening Sunday Night Football game on NBC this season.

The most obvious change is the addition of Peyton Manning to the Denver Broncos this offseason. Tim Tebow is long gone. John Elway, John Fox and Broncos’ offensive coordinator Mike McCoy are salivating over the prospect of one of the NFL’s greatest quarterbacks to play for the Broncos team that made huge strides in 2011, full of young talent and budding stars.

Manning will improve the offense considerably. The wide receivers will be challenged and spurred on by Manning’s leadership skills and expectation of perfection every play. Adding Manning will hopefully reduce the number of three-and-outs that characterized the time Tebow spent at quarterback for Denver. Look for Manning to extend drives and orchestrate the offense in a way that keeps the defense off the field and otherwise fresh.

Manning’s presence in the pocket will also improve the average yards running backs Willis McGahee and company will get on the ground as most teams will likely be playing the pass more often than not. Manning also likes the short routes. Ronnie Hillman, Knowshon Moreno and Lance Ball are very capable receiving running backs who can all make spectacular plays out in the open, something McCoy will look to exploit early and often. Denver’s new additions at tight end Jacob Tamme and Joel Dreessen will also be busy corralling passes over the middle from Manning.

The offensive linemen will be under the microscope this season as they take on the task of protecting Peyton even if there aren’t any concerns about his surgically repaired neck. Center J.D. Walton faces the biggest pressure to gel with Manning, make the right line calls and really lead on the line in 2012. The team is carrying three centers on the active roster and just picked up a fourth on the practice squad, adding rookie Quentin Saulsberry from Mississippi State.

As with any team, the quarterback is the most important player on the field. And losing your starting QB will doom the rest of the season to failure, especially with a guy like Caleb Hanie backing him up. Just ask the Chicago Bears.

The Broncos’ defense is getting a better mix of young and old all throughout the roster. The Broncos need young safeties Rahim Moore and Quinton Carter to continue to show improvement in coverage and tackling in 2012. Chris Harris beat out the veteran Drayton Florence who was released from the team and must continue to prove that he is the best option at nickelback for Denver.

The Broncos likely have one of the best pass-rushing attacks in the NFL with the trio of Von Miller, Elvis Dumervil and rookie Derek Wolfe. Miller must return to the form that he displayed in the first three quarters of the 2011 season and take it to the next level this year.

Run defense and linebacker play will likely be the most suspect area on defense for the Broncos. The run defense has been deplorable in recent years and has to improve to put the ball in Peyton Manning’s hands more often. The suspension for PED use this offseason of D.J. Williams has been a consistent distraction—and filling the hole he leaves during the six-game ban is a major concern. Joe Mays at middle linebacker needs to take the next step and improve his tackling, run defense and even coverage on crossing slot receivers and tight ends up the seam.

The Pittsburgh Steelers are healthier now compared to the postseason matchup they lost to end their 2011 season against the Broncos. The Steelers were missing center Maurkice Pouncey and safety Ryan Clark, among others.

Linebacker LaMarr Woodley was coming off a hamstring injury and running back Rashard Mendenhall injured his knee in Week 17. Each of those players should be active for the season opener against Denver in 2012. During the game, the Steelers lost star defensive end Brett Keisel to injury as well.

Pittsburgh is “healthier” because of the numerous injuries suffered during preseason this year by several young players on the Steelers roster. James Harrison, a monster on defense, looks set to take the field Sunday night against Denver as well after undergoing arthroscopic knee surgery last month. James Harrison’s backup Jason Worilds, a 2010 draft pick, had surgery on his left wrist this offseason that looks like it will be healed in time for the Broncos game on Sunday.

Stevenson Sylvester, backup to Larry Foote at inside linebacker, tore his MCL in his knee and is out another couple of weeks, reported Behind the Steel Curtain a few weeks ago. The injury meant that rookie inside linebacker Sean Spence, a third-round draft choice by Pittsburgh would fill in as backup at ILB and be a special teams contributor this season for the Steelers. That was until he tore multiple ligaments in his left knee in the preseason finale on Thursday against the Carolina Panthers.

At nose tackle, Casey Hampton is coming off a knee injury in 2011, which he had surgery to repair during the offseason. He also had surgery on his elbow. Ryan Clark will again be held out of the game in Denver because of a sickle cell trait condition, which hospitalized him after a game in Denver in 2007 and ended up resulting in the removal of his spleen and gallbladder.

Another huge injury the Steelers suffered this offseason was to their first-round draft choice right guard David DeCastro. DeCastro suffered a torn MCL and dislocated knee in the team’s third preseason game against the Buffalo Bills. He will go on the team’s injured reserve (IR) list to start the season, but the team hopes to re-activate him mid-season under new rule changes allowing the team to re-activate one player from IR as long as they were previously designated to be re-activated when entered on IR.

Good news for the Steelers was the arrival of Mike Wallace, the speedy receiver who signed his franchise tender to join the team. Wallace was welcomed back with open arms after holding out during the entire preseason workouts. Wallace may be a bit rusty (and more easily winded) in his return to the lineup next Sunday in the Mile High City, where the air is thin at 5,280 feet making conditioning a top concern.

One of the team’s newest weapons on offense is their fifth-round draft pick Chris Rainey, who is tough to tackle in open space. He is a running back, wide receiver and kick returner. Just get him the ball in open space.

Despite some of the setbacks this offseason, the Steelers are hungry and they will be gunning for the Broncos in Week 1. The Broncos need to wrap up when they are tackling and move the ball against a Steeler’s defense that has been consistently near the top to begin the Manning era in the win column this weekend.