On the morning of January 10, 2009, you might very well have woken up believing that the Tennessee Titans were the best team in the NFL. After starting 10-0, the Titans lost a pair of meaningless games and finished 13-3, giving them the best record in football. They led the league in point differential and were second in weighted DVOA when they chose to shut things down and prepare for the playoffs. While they had quarterback barely covered with Kerry Collins, their one-two punch of Chris Johnson and LenDale White had blossomed as the season went along, and with buzz-worthy head-coaching candidate Jim Schwartz coordinating a defense led by the seemingly unstoppable Albert Haynesworth, it would be fair to think that the Titans had every reason to look forward to the next few seasons.

They lost to the Ravens in the playoffs that day, 13-10, in a game in which Johnson got hurt and missed the second half. Since then, the Titans are 25-29, with one winning season built upon a remarkably easy schedule and a gift from the Texans in Week 17. Just five of the 22 starters who suited up for the Titans against the Ravens that day made it to the starting lineup in Tennessee’s narrow win over Pittsburgh on Thursday night, and it’s hard to argue that any one of the five look better for the trip.

What’s happened to the Titans? While they had to endure the final throes of the Vince Young saga and extricate themselves from their long-standing relationship with Jeff Fisher, the real difference between that dominant Tennessee team and the one that’s struggled over the past few seasons is more subtle. Spot-for-spot, that Tennessee team had the best set of lines in football. This Tennessee team doesn’t have the worst set of lines in the league, but they’re not far off, either.

By lines, I mean their five starters on the offensive line and their four starters up front on defense, and the glory days of that nine-man unit start with the aforementioned Haynesworth. For whatever trash you want to talk about Haynesworth and his assorted physical and personal maladies, if you go back and let it color your perception of the player’s performance before he left Tennessee, you’re mistaken. During the 2007 and 2008 seasons, Haynesworth was the best defensive player in football. Bar none. In 2007, Haynesworth was the leader on one of the best half-season defenses in the over-20-year history of DVOA before suffering an injury, one that coincided neatly with a dramatic decline in Tennessee’s defensive performance. In 2008, he was the only All-Pro or Pro Bowler on one of the league’s best front sevens, leading the team in sacks despite playing defensive tackle and missing two games due to injury. And Haynesworth had help; he was accompanied on the line by the likes of Kyle Vanden Bosch and Jevon Kearse, with promising situational players like Jacob Ford, Jason Jones, and William Hayes behind them. Tennessee had the best defensive lineman in football and, in all likelihood, the deepest group behind him.

The offensive line was no slouch, either. It featured an All-Pro left tackle (Michael Roos) and center (Kevin Mawae), the latter of whom missed the Ravens game with an injury. The team’s three other starters were all either 26 or 27, with right tackle David Stewart notable as a possible future Pro Bowler. They had been a dominant run-blocking unit that had also kept the limited Collins from taking too many sacks; after taking a sack on 5.7 percent of his dropbacks before the 2008 campaign, Collins was sacked on an astoundingly low 1.9 percent of his pass plays in 2008. With eight sacks across 15 starts, Collins basically took one sack every other game.

If anything, the offensive line got even better in 2009. Collins’s sack rate went up, but only to 2.7 percent; Vince Young took two-thirds of the snaps, and despite his propensity for scrambling his way into trouble, Young was only sacked on 3.4 percent of his pass attempts, which was almost half his career rate. Of course, that was also the year in which Johnson broke out as a superstar and ran for 2,006 yards and 14 touchdowns. Many of those runs came on huge plays, but Johnson was able to get the lanes for those huge plays at the line of scrimmage by virtue of his offensive line. Over that two-year stretch, you could also make a case that they were the best offensive line in football.

Since then, though, the line that grew together grew old, mostly together, until it fell apart. Mawae wasn’t the same guy in 2009; he made the Pro Bowl on scholarship and wasn’t offered a starting job in free agency, leading to rumors of collusion regarding his role as an NFLPA leader before his eventual retirement. The Titans expected to replace him with utility lineman Leroy Harris without missing a beat, but Harris failed to play at a starting-caliber level and has bounced around the interior of the Tennessee line, playing poorly wherever he lands. The team was forced to move Amano to center, where he was competent, but he’s missed this entire season with a torn triceps. They now have the ancient Steve Hutchinson manning left guard. Right guard Jake Scott is out of football, and the tackles — Roos and Stewart — might have peaked in 2009, even if they remain competent.

The scarier thing is that the Titans just haven’t invested anything in improving their offensive line over that time frame beyond the three-year, $16 million deal they gave to a 34-year-old Hutchinson this winter. Since 2008, they’ve invested just three draft picks in four years on offensive linemen, the highest of which was a fourth-rounder on the unforgettable Troy Kropog. Even if the Titans believed that they wanted to stick with their core on the offensive line and invest elsewhere, it seems incredible that they haven’t even spent as much as a third-round pick on an offensive lineman over the past four years.

The defensive line has fallen apart more quickly and noticeably. Hindsight might have proven the team right to let Haynesworth go, but they’ve never been able to replace the majority of his production since he departed for Washington. Vanden Bosch left shortly thereafter for Detroit, Kearse retired, and fourth starter Tony Brown is out of football. And all that depth the Titans had during the Schwartz era became mostly useless once the team got rid of the wide-nine scheme favored by legendary defensive line coach Jim Washburn, who is now in Philadelphia. Ford is out of football, Hayes is on the bench in St. Louis, and Jones washed out as a starter in Tennessee before moving to the Seahawks as a backup. Jason Babin emerged as a possible pass-rusher for the team after a 12-sack campaign in 2010, but he followed Washburn to Philadelphia last year and promptly posted 18 sacks, which was only four fewer than the entire Tennessee defensive line combined.

Instead, the Titans have attempted to rebuild the line and failed. At the centerpiece of that movement is end Derrick Morgan, who represents the sole first-round pick over the last five years that the Titans have used on a player who doesn’t line up at a fantasy football position. Although he missed most of his first year with a torn ACL, Morgan just hasn’t produced to the level resembling a first-round pass rusher when he’s been on the field, as he’s accrued just five sacks in 24 games played over his first three seasons as a pro. The Titans took Morgan just one selection after the Giants grabbed Jason Pierre-Paul from South Florida. You can’t guarantee that JPP would have developed the same way had he been nurtured somewhere outside of the Meadowlands, but that hurts.

The rest of Tennessee’s line consists of mediocre parts. The Titans’ other swoop into free agency this offseason saw them grab pass rusher Kamerion Wimbley from the Raiders, who was anonymous yesterday and has just one sack in five games. Last year’s leading pass rusher was interior situational lineman Karl Klug, who doesn’t have a sack this year, and before the Steelers game, Tennessee ranked dead last in pass defense DVOA. Their combination of Jurrell Casey and Sen’Derrick Marks on the interior has helped produce a merely below-average run defense, but the pass rush has fallen apart and taken the pass defense down with it.

Don’t be fooled by the way they looked against a group of backups and hobbled linemen at home against the Steelers last night; this defensive line is still very, very bad. And while Johnson was able to make some hay for the second time this season, the shocking number of 13-carry, 16-yard games he’s had over the past two seasons isn’t entirely a product of his new contract and some supposed lack of desire; it’s simply really hard to gain a lot of yardage when you’re desperately searching for space in your own backfield. Tennessee’s offensive line ranked dead last in Adjusted Line Yards last season and was all the way up to 31st before the Steelers game this year; they’ve turned into one of the league’s most dismal units, especially on the interior.

This is what happens to great teams, though. The thing they perceive to be a strength eventually becomes a weakness, even if they keep the same personnel around for years and pay them like they’re superstars. Think about the way the Dallas offensive line crumbled a couple of years ago. Heck, if you want to base it on last night, look at what’s happening to the Steelers secondary right now. For the Titans to return to their prior glory, they need to get away from spending first-round picks on wide receivers and giving overstuffed contracts to running backs, and get back to investing in their lines.