Free agency is an opportunity to fill holes in the roster using veteran players with NFL years of service ahead of the NFL Draft.

The draft is still the preferred way of roster building for most teams. It's cheaper, and allows teams to mold developing players to their liking.

Free agency is the more expensive option, but it's a good supplement to the draft.

Let's take a look at the five biggest positions of need for Detroit heading into free agency:

1. Pass rushing help

Aside from defensive end Trey Flowers and linebacker Devon Kennard, who tied for the team lead with 7.0 sacks last season, the Lions got very little from the rest of the roster in terms of pass-rush impact. In fact, no other player on the team had more than 2.0 sacks.

This is a deep and talented free agent pass rushing class. There are five edge rushers in this free-agent class who recorded at least 10.0 sacks last season.

It's not all about sacks, but also consistently affecting the quarterback with pressure. There were 18 teams last year with more quarterback hurries than Detroit's 183 (per Pro Football Focus). Pass defense is a marriage of rush and cover, and the rush simply didn't do its part consistently enough last season for the Lions.

2. Interior defensive line

The Lions released Damon Harrison Sr. last month. A’Shawn Robinson and Mike Daniels are scheduled to become unrestricted free agents next week. That leaves Da'Shawn Hand, Kevin Strong, John Atkins, Frank Herron and Olive Sagapolu under contract. Hand is a terrific young talent, but he's missed 16 games due to injury in his first two seasons.

What was expected to be a strength on defense heading into last season, the defensive tackle group as a whole was rather disappointing. Detroit needs more talent, more depth and more production from that group. A little more pass-rushing prowess wouldn't hurt, either.

This is a strong group of free-agent interior defenders, maybe the deepest of any position in free agency, which could be good news for the Lions in the coming weeks.

3. Cornerback

This is a need even if Darius Slay returns. In that scenario, the Lions will have Slay, Justin Coleman, Amani Oruwariye, Jamal Agnew and Michael Jackson under contract. Mike Ford and Dee Virgin are exclusive rights free agents. Oruwariye showed some good things as a rookie, but is he ready for a starting role in just his second season? Detroit seems likely to be in the market for a cornerback in free agency and possibly the draft, even if Slay, 29, returns.

If the Lions trade Slay, move this up the priority list.

4. Backup quarterback

The numbers don't lie. Detroit was 0-8 playing without Matthew Stafford last season. Jeff Driskel (0-3) is a free agent, while David Blough (0-5) is under contract. Both Driskel and Blough did some good things filling in for Stafford last season, but quarterbacks are ultimately measured on wins and losses.

The Lions could be on the lookout for an established veteran – and there's a number of those on the free-agent market this year – to compete with Blough and Kyle Sloter for Detroit's backup job.

Stafford is expected to be 100 percent healthy and ready to go for the start of the offseason training program later next month, but last year proved the Lions need a better backup plan in place in case Stafford suffers an injury.

5. Offensive line

The release of Rick Wagner over the weekend has left a void at the starting right tackle spot. The Lions could turn to third-year tackle Tyrell Crosby, who has been the team's swing tackle the last two years, and has played pretty well when given an opportunity to do so, but free agency is also an option.

It seems more and more likely veteran right guard Graham Glasgow will test free agency and potentially sign elsewhere. Starting left guard Joe Dahl is under contract in 2020, but Glasgow, Kenny Wiggins and Oday Aboushi, who all played significant roles at guard last season, are unrestricted free agents.

The Lions could look to re-sign Wiggins, or potentially look elsewhere in the free agent market, but the numbers on the roster are pretty thin at guard right now.

The team likes the trajectory Beau Benzschawel is on, but is the undrafted rookie from a year ago ready to start in year two?

The Lions could potentially be without the starting right side of their offensive line from last year, meaning they'll need to address the line either in free agency or the draft.