Glover Quin is a Pro Bowl safety for the Detroit Lions whose 30 tackles are second most on the team this season. But there's another player stat that is arguably more noteworthy: According to ESPN, Quin has been investing 70 percent of his salary in companies since he entered the NFL in 2009.

Quin and his financial advisor estimate that within five years, his portfolio, which ranges from health apps to pet companies, will match his NFL earnings--$21 million.

The player started out investing in publicly traded firms. Shortly after signing a five-year contract with the Lions in 2013, he began investing in higher-risk ventures, putting 10 to 20 percent of his wealth into startups.

One of the main criteria he uses when deciding whether to invest: Can this company change and better the world? "If we feel that way," Quin told ESPN, "and we believe in the company and we believe in the direction that they are going and the people that are behind it, I feel a lot more comfortable."

Though Quin keeps much of his portfolio private due to contractual agreements, three of the startups he's invested in are PeerWell, a fitness app that helps patients prepare for surgery and recovery; Health Warrior, which makes food out of chia; and PawTree, a company that offers customized nutrition for pets. In the off-season, he'll sometimes analyze two to three potential new investments per month.

Quin majored in business at the University of New Mexico and hired former classmate Humble Lukanga as a financial advisor after being selected in the 2009 NFL draft. Lukanga, whose client list includes other NFL players, recommended the 70/30 invest/spend split. Quin adopted the strategy immediately and still uses it to this day.

The player's yearly salary now stands at more than $4 million. But when he was under his much-less-generous rookie contract, the 30 percent spending allotment meant he and his family lived on about $6,000 per month.

ESPN says Quin gets chided by teammates for his relatively minimal spending--in addition to his careful budgeting, he still drives a Yukon Denali he bought when he signed his first NFL contract. "First, you form habits," he says, "and then habits form you."

So even if he hangs it up after his current contract ends in 2017, he has a nice retirement plan in place.

Quin isn't the only high-profile NFL player with disciplined spending habits. When Seattle Seahawks star running back Marshawn Lynch retired earlier this year, it came to light that Lynch hasn't spent any of the $50 million he made in his playing career, instead living off money from endorsements.