Coaching matters in the National Football League. The quarterback is the most important player on the field, but without a strong coaching staff, even the most talented ones can struggle. The successful combination of the two is a very significant driver of your franchise's long-term success.

The Pittsburgh Steelers, under Mike Tomlin, are a perfect example. Tomlin joined the Pittsburgh Steelers in 2007, the fourth year of Steelers QB Ben Roethlisberger's illustrious 18-year career with Pittsburgh. During the fifteen years Tomlin and Roethlisberger led Pittsburgh into battle, Roethlisberger became only the fifth player in history to throw for over 64,000 career yards. They won a Super Bowl together, while Tomlin set one of the most impressive records a coach can earn by registering 15 consecutive non-losing seasons.

Now with Roethlisberger retired, Tomlin marches the Steelers into a new era. But can he continue his streak with Mitch Trubisky under center?

Pittsburgh's problems extend past quarterback

Mike Tomlin has done a masterful job getting the most of his players during his career. However, coaches need players, and there are enough red flags on this roster to signal this is the season the bottom falls out for Pittsburgh. Here are three key metrics where Pittsburgh ranked at the bottom of the league that I think will highlight the challenges they will face this season.

• Offensive Line (30th)

• Explosive Play Rate (29th)

• Defense (26th in EPA per rush allowed)

Pittsburgh's an auto-fade if you believe games are won in the trenches. Last season, the Steelers defense allowed a league-leading 143.8 yards rushing per game, and they didn't do much to improve in the offseason. They have gradually moved away from being the tough, physical team that will impose their will on their opponents and spent their top two draft picks on offensive skill position players for the second straight season.

Unfortunately, without a strong offensive line, those skill position players will have a tough time succeeding. Last season, RB Najee Harris was one of the league's leaders in broken tackles and was still held to only 29 yards in Pittsburgh's 42-21 playoff loss to Kansas City. The Steelers' free-agent signings of Mason Cole and James Daniels do little to improve the outlook of one of the worst offensive lines in football. The Steelers unit was recently ranked 30th by Sharp Football for this upcoming season.

That leads me to the most critical position on the field, the quarterback. Ben Roethlisberger had his limitations last season, but they will miss his experience. Putting Trubisky, a quarterback who has struggled with decision-making, behind this offensive line will be a problem. Even if Steelers offensive coordinator Matt Canada looks to roll Trubisky out of the pocket, it won't be easy with two tackles that rank outside the top 60 at their position. It's also not ideal for a rookie quarterback, so I don't expect the Steelers to move to Pickett early in an attempt to save the season.

Typically, when a team comes off a year where they ranked 28th in yards per play, the only way to go is up. But I am not sold that's going to be the case this season with the Steelers offense under either quarterback. In his rookie year, Pickett will likely have the same challenges as Trubisky.

Pittsburgh Steelers Under 7.5 wins (-110)

It's a good bet this season marks the end of an incredible run by Mike Tomlin. Based on projected win totals, the Steelers have the fourth-most difficult schedule. They open the season as 6.5-point dogs against the defending AFC champion Cincinnati Bengals. They also face the Buffalo Bills and Tampa Bay Buccaneers in the first six weeks. The Steelers are -200 to finish last in the AFC North and -375 to miss the playoffs.

The betting market is telling us this isn't a good football team, and I am willing to listen. Pittsburgh is the projected favorite in only four games the entire year, so having the total win set at 7.5 creates value on the under. I have watched Andy Reid go 4-12 in his last season in Philadelphia and also saw how a below-average quarterback room can make Bill Belichick look human. You can only coach them up so much. Betting against Mike Tomlin hasn't paid off in the past, but it's a new era in Pittsburgh. Mitch Trubisky, behind a poor offensive line, sounds like a good bet to go 7-10 or worse.

Stats provided by Clevanalytics, Sharp Football, PFF.com, Football Outsiders