ENGLEWOOD, Colo. -- For two consecutive seasons the Denver Broncos have adjusted, lamented, switched game plans, juggled lineups and generally wrung their hands plenty because of the way the team's offensive line has performed.

Last season, a veteran presence like Ryan Harris helped the group gather itself down the stretch and the Broncos' front likely played, in what was remarkably good timing, its best game of the season in the team's Super Bowl 50 win.

This season, with Harris gone in free agency -- he has been on injured reserve for the Steelers -- the Broncos haven't rebounded. In fact, several personnel executives in the league quickly say what many among the Broncos' faithful have believed throughout the season: Offensive line is the biggest on-field reason the team isn't in the playoffs.

Coach Gary Kubiak has acknowledged the struggles, often offering "they need to play better and they know that. But we have a lot of people like that, in that position. We all need to do better and that starts with me because ultimately it's on me."

The Broncos will close out a disappointing 2016 season Sunday against the Oakland Raiders. The season marks the Broncos' first playoff miss since 2010.

Kubiak has already said this week "is about 2017" and the Broncos are expected to aggressively dive into the offseason to remake the roster spots they believe need it. The offensive line figures to top the list and there could be some changes on the coaching staff as well. The Broncos have made an investment in recent years on the offensive line in the draft and the open market, but they don't have the success to show for it.

There was some thought that the Broncos could get by on offense, after all they won a Super Bowl thanks to their elite defense. That notion officially expired in the muck of Arrowhead Stadium Christmas night.

In the six previous drafts John Elway has overseen, the Broncos have selected eight offensive linemen. Four of those linemen -- Orlando Franklin (2011 draft), Michael Schofield (2014), Matt Paradis (2014) and Max Garcia (2015) -- started more than eight games for the team to this point.

Franklin left in free agency in 2015. Of the remaining three, Paradis, who is facing offseason hip surgery, could be the only player who is close to being in the plans as a starter next season. The Broncos like Schofield's versatility, though, and how he has worked since his on-field struggles in 2014, most notably Khalil Mack's five-sack game in Denver.

The Broncos also invested in free agency last offseason, signing tackles Russell Okung and Donald Stephenson. Fifteen regular-season games later, it's clear the Broncos face decisions on both in the coming months given both have not performed as hoped.

Stephenson had a five-penalty game in Kansas City and he is the most penalized player on the team, with 11. Okung is just behind with 10. Okung signed what is potentially a five-year, $53 million deal last offseason, but the final four years of the deal only kick in if the Broncos pick up a $1 million option bonus by March 9. If they don't pick up the bonus by that date, Okung is a free agent and the final four years of the deal are voided.

Stephenson signed a three-year, $14 million deal last March. He got a $3 million signing bonus to go with a $3 million base salary this season. However, his 2017 base salary ($4 million) becomes guaranteed on March 13 if he's on the roster. If the Broncos elect to release Stephenson after the season, they will get a dead money charge of $2 million for 2017, which is less than the salary guarantee.

Fueling it all is the disappointment of the Broncos' running game and that the Broncos believe they have two viable starting quarterback prospects in Trevor Siemian and Paxton Lynch. But they feel that the growth of the two quarterbacks will be greatly stunted without quality pass protection. More than one personnel executive in the league has said the punishment Siemian took in the pocket this season, especially in his 368-yard day against the Chiefs last month, showed up in his footwork in his last two starts.

The Broncos are tied for the fifth-most sacks surrendered in the league -- 40 -- and Siemian took a pile of hits in November and December. So, even as Kubiak puts the blame on his own shoulders, there figures to be plenty of attention on not only both of the team's quarterbacks in the offseason to come, but perhaps even more on the guys in front of them.

"We're not playing well enough offensively, I'm responsible for that," Kubiak said this week. "I have to do a better job. We're not getting enough out of our opportunities so I take responsibility for that."