If Matt Nagy is serious about looking in the mirror this week, he will see a Bears coach responsible for the offense becoming too soft.

The Bears offense is the doughy guy at the gym with the love handles, vowing to change but lacking the discipline to follow through.

Only Nagy can force the offense from featuring so much finesse and emphasize the run. Under Nagy, the Bears flirt with the running game but never fully commit to it. More than anything, that fear of commitment has prevented the offense from finding football bliss.

A disappointing offensive line has been a bigger problem through five games than quarterback or kicker, the positions that commanded so much attention for so long.

As this season comes to a crossroads after arguably the worst loss since Nagy arrived, it’s up to the offensive guru to coach the Bears through a crisis largely of his own making. Running the ball requires a mindset the pass-happy Bears and their schematically gifted play-caller too often look incapable of adopting for four quarters.

The Bears average 80.6 rushing yards per game, 26th in the NFL. The six teams ranked below the Bears have combined for two victories. They rank 20th in rushing attempts per game with 23.6.

Look around at what’s working in the league in 2019. What’s new feels old: Running the ball well matters again. The 49ers, the NFC’s biggest surprise at 4-0, gashed the Browns for 275 rushing yards Monday night and lead the league with a 200-yard average.

A night earlier, NBC essentially ran a three-hour commercial for power football as the Colts upset the Chiefs 19-13 on the strength of their running game. In a game that featured reigning MVP quarterback Patrick Mahomes of the Chiefs, Colts All-Pro guard Quenton Nelson stole the show.

The Bills, 4-1, believe in the correlation between their quick start and the NFL’s seventh-best rushing attack. And nobody in Chicago needs reminding why the Raiders, who just steamrolled the Bears behind a massive offensive line, sit at 3-2 under old-school coach Jon Gruden.

As Hall of Fame football writer Rick Gosselin of the Dallas Morning News pointed out, Week 5 featured nine 300-yard passers and their teams went 4-5. In contrast, there were eight 100-yard rushers and their teams went 7-1.

David Montgomery, the Bears’ leading rusher, is on pace to gain 720 measly yards. Montgomery looks like the real deal, but the sample size remains too small to judge — and his room to run has been too scarce to tell. That onus falls on the linemen trying in vain to make the room. A good question is whether an NFL offensive line can improve during a season. The answer is the Bears’ success depends on it. As elite as the defense is, the offense can increase the margin for error by imposing its will.

Should Nagy use the time off this week to tweak the lineup on an underachieving offensive line? Isn’t this the right time to challenge the group to improve — or else? How often have you noticed the Bears offensive line firing off the ball and controlling the line of scrimmage?

The Bears have every reason to expect more because they’re not getting enough bang for their buck.

Left tackle Charles Leno, guaranteed $21.5 million when he signed a four-year, $38 million contract extension in August 2017, leads NFL offensive linemen in penalties with eight — including four holding calls. Right tackle Bobby Massie, who signed five-year, $40 million extension last winter with $15.8 million guaranteed, has been spotty. Right guard Kyle Long looks like a shadow of his former self because of a balky hip. Left guard Cody Whitehair, fresh off signing a $52.5 million extension after training camp with $27.5 million in guarantees, at times still appears to be adjusting to his move from center. Center James Daniels shows promise but occasionally misses a block or assignment that reminds you this is his first full NFL season as a starter.

Offensive line coach Harry Hiestand, one of the best in the business, can only do so much before the Bears consider alternatives. Rashaad Coward, a converted defensive tackle with raw skills, flashed ability at guard filling in for Long. Ted Larsen, a veteran who offers an edge missing among Bears starters, could enter the mix after returning from a knee injury.

The most intriguing option might be Alex Bars, the undrafted rookie from Notre Dame who turned down a chance to join the Patriots’ 53-man roster to stay on the Bears practice squad, according to the Tribune’s Brad Biggs. Bars impressed the Bears during preseason when pressed to play left tackle and could push Leno if the Bears want to light a fire under the veteran.

When the Bears reconvene, it will mark the time for the offensive line to assert itself so an offense with no identity finally develops one. It will be important for Nagy to pare down his voluminous playbook to pound the running game. If the offensive line plays up to its potential more than it has, it will mitigate the importance of quarterback Mitch Trubisky’s health or fill-in Chase Daniel’s ability.

Every quarterback at every level benefits from an effective play-action passing game. The offensive priorities should be finding enough touches for wide receiver Allen Robinson, running back Tarik Cohen and Montgomery — the three biggest playmakers — and not worry so much about devising plays to get everyone involved. Tight end Trey Burton remains a rumor in the offense and flashy wide receiver Anthony Miller needs to prove the pros outweigh the cons when targeting him in the passing game.

Simplify the goals, spread out the defense and smash opponents in the mouth — the way the Raiders just did in London.

For the Bears’ sake, hope that it left an impression on Nagy.

David Haugh is a special contributor to the Chicago Tribune and co-host of the “Mully and Haugh Show” weekdays from 5-9 a.m. on WSCR-AM-670.