The NFL free agency negotiating period begins Monday, and the market will open at 4 p.m. Wednesday. The Lions have been active in free agency under Bob Quinn, too, handing big contracts to players like receiver Marvin Jones in 2016 and offensive linemen T.J. Lang and Rick Wagner in 2017. MLive will mull possible fits for 2018 in the coming days. Today: Defensive backs. Previously: Offensive line | Running backs | Receivers/tight ends | Quarterbacks

Under contract for 2018: Glover Quin ($6.5 million cap hit), Darius Slay ($6.1 million), Quandre Diggs ($1.9 million), Teez Tabor ($1.1 million), Miles Killebrew ($775,000), Jamal Agnew ($620,000), Stefan McClure ($555,000), Charles Washington ($555,000), Adairius Barnes ($555,000), Desmond Lawrence ($480,000), Rolan Milligan ($480,000)

Pending free agents: Don Carey, D.J. Hayden, Nevin Lawson, Tavon Wilson

Breakdown: Their pursuit of Richard Sherman died before he ever set foot in Detroit, but have no fear. The Lions are deeper in the secondary than just about anywhere else. Darius Slay is in the middle of his prime, and locked up through 2020. Glover Quin continues to be one of the best players nobody talks about in the NFL. And Quandre Diggs was so good in his month at safety that he could head into 2018 as the full-time starter alongside Quin.

With that said, the Lions do have question marks at the position, most notably who starts opposite Slay at cornerback? Lawson is a free agent, and there's been no indication about his future. As GM Bob Quinn suggested at the combine, the Lions have decent internal options. Teez Tabor didn't play much to open last season, which alarmed some who were expecting more out of the second-round pick, but the truth is it was always the plan to bring him along slowly as he adjusted to the nuances and speed of the pro game. He played an expanded role down the stretch, and will have a shot to compete for the job.

Unless, of course, Detroit makes a splashy free-agent signing. And the club's reported interest in Sherman could be a sign it's willing to pour more money into the position.

The other major unknown at this point is who will play in the slot. Diggs handled the assignment most of 2017, and could return there. But if he stays put at safety, as many suspect, Detroit will need to find a successor. Hayden filled the role down the stretch last year, but he's set to hit free agency in three days. Lawson is too.

With that, Jamal Agnew is probably the No. 1 slot on the roster right now. He played sparingly on defense as a rookie, but he's quick as rain and continues to receive high praise from Quinn for his development. But the Lions are going to want more bodies for the position, and could target somebody with more experience to balance with Agnew's youth.

At safety, the Lions could bring back Tavon Wilson, who started alongside Quin much of the last two years before suffering that injury late in 2017. He might have been Wally Pipp'd by Diggs, but if he doesn't find suitors on the open market, he could return to Detroit and provide some quality depth at the position. Other than that, there really isn't much drama going on at safety. Such is life with Glover Quin.

Here's a look at what free agency can offer:

Malcolm Butler, 28, Patriots: The hero of Super Bowl XLIX did not play in Super Bowl LII. That's befuddling, of course, and has sparked questions about whether he'd even be willing to reunite with his former defensive coordinator. But Matt Patricia insists they have a great relationship -- which, if true, could help the Lions woo Butler if they're interested in pairing Slay with another first-rate corner. "I would characterize my relationship with Malcolm as extremely strong," Patricia said. "I love Malcolm a lot. Like all my players, he's like one of my sons. I want to make sure that he does everything to the best and I hope the best for him. And that's really all I'm going to say about Malcolm."

Patrick Robinson, 30, Eagles: If the Lions aren't comfortable rolling into 2018 with an unproven Agnew in the slot, they just might be comfortable with this guy. He allowed a passer rating of just 65.2 when he was lined up inside, which was third best in the league and No. 1 among players who can hit free agency this week. And don't forget about that pick-six in the NFC championship either, which helped turn the tide of that game.

Brent Grimes, 34, Buccaneers: Already one of the oldest defensive backs in the league, there's no telling how much tread is left on the tires. Then again, he still had enough tread left last year to be Tampa's best cornerback. He picked off three passes and held QBs to a rating of 73.8 on balls thrown his way. That was 20th in the league, and better than every Lions cornerback not named Slay. So it sure seems like he could help Detroit, and would come a lot cheaper than someone like Sherman or Butler too.

Kenny Vaccaro, 27, Saints: One of the beauties of a versatile player like Diggs is it gives the Lions options. They could sign a slot corner if they find a good one, then move Diggs to safety. But if they have better luck at finding a safety, they could always leave Diggs in the nickel, where he ranked fourth in the league last year according to ProFootballFocus. If the Lions do elect to sign a starting caliber safety, Vaccaro is a fit worth watching. He's mostly played strong safety, which would complement what Quin does as the free safety, but he has the versatility to play either as well. And, again, this new regime is going to be all about versatility, especially on defense.

Others to watch: CB Ross Cockrell, CB E.J. Gaines, CB Robert McClain, S Morgan Burnett, S Corey Graham