Two years ago, Ohio State couldn't keep Raekwon McMillan off the field despite its wealth of defensive talent.

The Buckeyes returned a veteran starting middle linebacker in Curtis Grant, but McMillan was a five-star recruit with NFL size who enrolled early and offered a Big Ten-ready skill set. While he didn't start a game, he finished the season with 54 tackles and, according to his official Ohio State bio, played more snaps than Grant in nine of the Buckeyes' 15 games. He represented the future of the Ohio State defense.

Now, McMillan has a full season under his belt as a starter, and he is suddenly the Ohio State defense's last significant link to the past and the one player the unit cannot afford to lose. The other two returning starters on defense -- end Tyquan Lewis and cornerback Gareon Conley -- saw action as reserves during the title-winning season, but McMillan was the only regular impact player on the 2014 national championship team that will still be suiting up for the Buckeyes on defense this fall.

The 2016 season represents a changing of the guard for Ohio State, and while quaterback J.T. Barrett appears to be the most important player on the team, the stability offered by the return of McMillan comes close to matching Barrett, because the defense will define just how far Ohio State can go this fall just as much as, if not more than, the offense.

The presence of Barrett prevents direct comparisons to Urban Meyer's 2010 Florida team. In 2008, the Gators won the national title with a significant number of underclassman stars. They proceeded to go 13-1 the next season, losing the SEC title game to Alabama and thus ending up in the Sugar Bowl instead of the BCS title game, which was deemed a disappointment. With Tim Tebow and countless other stars gone, the Gators opened the next season ranked fourth in the AP poll but stumbled to an 8-5 finish. Florida's 2008-09 sounds a lot like Ohio State's 2014-15, with the same one-loss records but no championship repeat.

In 2016, Ohio State is charged with avoiding a similar fate to the Gators' 2010. In this case, though, Ohio State has a star quarterback returning in Barrett, unlike Florida did. While there are many unanswered questions at the skill positions around him, it's the progress of the overhauled defense that may determine whether the Buckeyes reload or fall back and rebuild. After all, amid an up-and-down season for the offense last fall, it was the defense that often provided stability, a few hiccups aside.

The Buckeyes' struggles last season were often overstated because of how high expectations were, and the defense in particular was excellent nearly every game, ranking sixth in yards per play allowed and posting the seventh-best defensive SRS (Sports-Reference's Simple Rating System) in program history, just behind a historically great Alabama team.

For stars in college football, the transition from up-and-comer to savvy veteran happens quickly -- especially when a lot of key veterans are leave at the same time -- and now McMillan is one of the most vital defensive players in the country. In 2014, Ohio State returned more proven talent than this year's team will, but the defense nevertheless had a lot of young players still developing into stars. That process of development happened over the course of the season, the Buckeyes overcoming issues in the first two months to emerge as a behemoth down the stretch. It wasn't just the play of quarterbacks J.T. Barrett and Cardale Jones that made the difference; it was the breakouts of rising stars like Joey Bosa, Vonn Bell, Darron Lee and Eli Apple.

Ohio State will be hoping for similar progression this fall with a unit that will look very different, with the loss of so many key players plus defensive coordinator Chris Ash, who is now head coach at Rutgers. Six defensive players were drafted, including Bosa, Apple and Lee in the top 20. Bell and defensive tackle Adolphus Washington went on Day 2, and linebacker Joshua Perry was drafted in the fourth round. Safety Tyvis Powell left early and went undrafted, and the Buckeyes also lose starting defensive tackle Tommy Schutt. Seven of the Buckeyes' top 10 tacklers are gone, with only one starter returning to each of the three defensive units.

Given the recruiting prowess of Meyer and his coaching staff, Ohio State has much higher expectations than any other program but Alabama could reasonably have following the mass departure of so many impact players. Last year, Buckeyes dominated Sports on Earth's preseason returning player ranking series. This year, the series will be much lighter on Buckeyes, although McMillan checked in as the No. 3 linebacker.

There are ample reasons for hope despite all the new faces. Lewis, not Bosa, led the Buckeyes with eight sacks last season. While that is partly because Bosa received so much attention from opposing offenses, the junior end can more than hold is own and is positioned for an All-Big Ten type season. He'll be joined in the pass rush by two other breakout players at end: Sam Hubbard, who had 6 ½ sacks as a freshman, and Nick Bosa, the five-star true freshman brother of Joey.

Ohio State has been dominant on the recruiting trail in recent years, and thus recent blue-chip recruits like linebacker Jerome Baker, cornerback Marshon Lattimore could join Hubbard in having breakthrough seasons. The new coordinator transition should also go smoother than it did on offense last year, with former Rutgers head coach Greg Schiano stepping as the lead defensive assistant, alongside longtime Ohio State assistant Luke Fickell and respected defensive line coach Larry Johnson.

The rebuilt unit will be tested early. Ohio State's first two games are at home against Group of Five teams, but these are no ordinary mid-major offenses. Bowling Green and Tulsa were two of the top-11 passing offenses in the country last season, explosive teams that put a lot of pressure on defenses. Bowling Green lost its coach (Dino Babers) and quarterback (Matt Johnson), but the Falcons and Golden Hurricane both have the ability to provide a strong early-season measuring stick for Ohio State's defensive progress. The Buckeyes will need it, considering their third game is a brutal road trip to defending Big 12 champion Oklahoma, who boasts a Heisman candidate quarterback in Baker Mayfield and running back in Samaje Perine.

As we saw in 2014, a nonconference loss won't end Ohio State's playoff chances -- particularly a road game against a quality opponent like Oklahoma -- but if the Buckeyes do lose in Norman, their margin for error in competing for a national championship is erased. The Big Ten schedule isn't exactly easy, either, with back-to-back road trips to Wisconsin and Penn State in October, Northwestern and Nebraska coming to Columbus from the Big Ten West and games at Michigan State and vs. Michigan to end the season. It helps to have Michigan State and Michigan -- the likely top two challengers in the Big Ten East -- at the end of the season at least, as it can be expected that the Buckeyes will improve significantly over the course of the fall.

Ohio State was fascinating to watch last season because it was a highly regarded defending national champion that went through a soap-opera season stacked with NFL-ready talent. Despite the turnover, this year's squad may be equally fascinating -- if not more -- as Meyer attempts to show just how quickly he can make recruiting success pay off on the field. The Buckeyes have gone from the surest preseason bet in the country a year ago to one of college football's greatest mystery teams.

In 2014, the Buckeyes won one national title ahead of schedule. It's a long, tough road to doing so again in 2016, but with McMillan at the center of a defense more likely to reload than rebuild, don't be surprised if Ohio State finds at least some of that '14 magic again to have a shot at more national glory, successfully bridging the gap between past and present.

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