You don’t know what you have until it’s gone.

That’s an old phrase that Purdue has learned over the last three weeks. Because for the past three games, the Boilermakers have been without one of their top defensive linemen and things aren’t going well.

For the past three seasons, Jake Replogle has been a staple for Purdue’s defense.

The defensive tackle started in all 12 games last year and has tallied 141 tackles and seven sacks in his career. Last season, he was a monster in the backfield, totaling 14 tackles for loss and was frequently chasing down quarterbacks.

He was Mr. Reliable. In 2015, he developed into Purdue’s top defensive lineman and was arguably the best defender on the roster. His image as one of the B1G’s top tackles and disrupters at the line of scrimmage continued to grow into 2016.

To start the year, Replogle was having another solid season. His numbers weren’t quite as good as they were a season ago, but he was still pressuring the quarterback and proved to be a difficult guy to block for offensive linemen. Though Purdue’s defense was still struggling, he was the anchor and seemed to provide stability when it was needed most.

So it’s no surprise that the Boilermakers defense started to decline after a 34-31 win over Illinois. Since that time, Replogle has been limited in practice and has missed games against Iowa, Nebraska and Penn State due to a concussion.

For Purdue, this has been no minor loss. Despite the drop in production according to most statistical categories, Replogle was still putting together an incredible season. One that was earning the senior captain recognition from multiple platforms nation wide.

Prior to his injury, Replogle was college football’s most productive defensive tackle in pass rush productivity according to Pro Football Focus:

In five games, Replogle recorded 28 stops, three tackles for loss and 1.5 sacks to go along with his 22 quarterback pressures. Those numbers helped land the senior on Pro Football Focus’s midseason All-American team, along with Alabama’s Jonathon Allen.

Returning that type of productivity would be a boost for Purdue’s defense, especially as it prepares for some of the top offensive weapons in the conference.

It starts this weekend against Minnesota, a team that had its best offensive performance of the year against the Boilermakers last season. And the Gophers have a pair of pretty talented ball-carriers, Rodney Smith (801) and Shannon Brooks (472), who can do some damage on a bad run defense. Quarterback Mitch Leidner has been inconsistent again this season, but his mobility could be hard for the Boilermakers to contain without Replogle to get pressure in the backfield.

Without a consistent disrupter at the line of scrimmage, Minnesota could have another big day offensively, especially on the ground. The combination of Smith and Brooks own the conference’s fourth-best rushing mark while Purdue’s defense ranks 13th in the B1G at defending the run, allowing 250 yards per contest.

RELATED: Why Has Wisconsin Been So Bad At Northwestern In The 21st Century?

And then things get even more challenging.

Northwestern’s offense is getting hotter by the week. Justin Jackson is the B1G’s second-leading rusher with 868 yards and six TDs. Clayton Thorson is continuing to blossom into one of the top quarterback’s in the league, throwing 10 TDs and two INTs in the Wildcats’ last four outings. That’s going to be a tough combination for Purdue to stop without its best defensive weapon.

Finding ways to stop running backs Corey Clement and Dare Ogunbowale from Wisconsin and slowing down a potentially potent passing game from Indiana’s Richard Lagow will be troublesome, too.

Replogle’s presence isn’t measured by just the numbers. When he’s been on the field, he’s drawn double-teams, opening up lanes for other guys to make plays. He’s got great quickness for a defensive lineman and reads the offense extremely well. It’s rare that you’ll see the senior out of position or miss many tackles in his direction.

That’s something that his freshman replacement – Lorenzo Neal – hasn’t been able to replicate.

Oct 8, 2016; Champaign, IL, USA; Illinois Fighting Illini quarterback Chayce Crouch (7) is pressured by Purdue Boilermakers defensive tackle Jake Replogle (54) during the third quarter at Memorial Stadium. Purdue beat Illinois in overtime 34 to 31. Mandatory Credit: Mike Granse-USA TODAY Sports

Defensive end Evan Panfil has stepped up since Replogle’s injury and has provided at least some spark of life to the defensive front. In the last three contests Panfil has collected six stops in the backfield, improving his total to 10.5 on the year, leading the team.

Nobody else has really made a huge impact.

Purdue lacked quality depth defensively to start the season. Injuries to Ja’Whaun Bentley, Da’Wan Hunte and now Replogle have been a leading cause of some of the biggest struggles on that end of the field. Getting off the field on third down (last in the B1G), Stopping the run (13th in the B1G), and giving up big plays (last in the B1G) have been issues since the first week.

Aside from Penn State’s poor third down conversion rate last weekend (2-of-9), those issues have been magnified since Replogle’s head injury.

Purdue’s defense wasn’t great with Replogle in the lineup. Cincinnati piled up 512 yards in the second week of the season and Maryland obliterated the Boilermakers on the ground, rushing for 400 yards en route to a 50-point outing. In the first five games of the year, opponents were averaging 431.6 yards per game. More than half of that total was credited to the rushing attack.

RELATED: Believe It Or Not, There Actually Is A Way For Purdue To Still Win The B1G

But the numbers have gotten worse.

In the last three outings, teams are averaging 480 yards of offense per game. The Boilermakers surrendered season-high point totals to Iowa (49) and Penn State (62). Both of those games were played in West Lafayette. Iowa’s LeShun Daniels Jr. and Akrum Wadley each had season-high rushing totals. Saquon Barkley had a career-best afternoon.

As long as Replogle is sidelined, those numbers aren’t going to look much better.

They may not improve even if he does return to the lineup. But over the past two seasons, he’s been the one consistent factor on the defensive line. Purdue’s defense is certainly better with their defensive tackle than it is without him.

Replogle’s status is still questionable heading into Saturday’s contest against Minnesota. If he’s cleared, Purdue’s defense will get the extra life it’s been lacking the past few weeks. If he’s still on the sideline, the Boilermakers may have trouble getting the Gophers offense off the field for a second-straight year.