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In the days leading up to the start of Lions training camp, Justin Rogers of The Detroit News will highlight the key position battles. Today: Backup defensive tackles.

Allen Park — The Detroit Lions used to have the most feared group of defensive tackles in the NFL, led by Ndamukong Suh and Nick Fairley. Those days are long gone, but the current top of the rotation remains capable of impacting a game.

Haloti Ngata was brought in via trade when Suh bolted for a bank-busting deal in free agency. One of the best players of his generation, Ngata is well past his prime at 33 years old, but remains effectively disruptive when he’s healthy.

Ngata may not get after the quarterback the way he used to, but opposing running backs won’t find much space running to his gap, and the five-time All-Pro has always shown a knack for getting his hands in the passing lanes.

Speaking of batted passes, rookie A’Shawn Robinson proved quite adept at knocking down throws at the line. His seven breakups were good enough for third on the team, behind only starting cornerbacks Darius Slay and Nevin Lawson.

Robinson, a second-round pick in 2016, has the potential to be an above-average starter for years to come for Detroit. The 22-year-old Alabama product already possess a well-rounded skill set and showed steady improvement during his inaugural season, earning starts in five of the team’s final six games.

Behind the starting tandem, the Lions have a hodgepodge of young veterans who haven’t lived up to their potential and an athletic, albeit undersized, rookie.

The Lions clearly like what they see Akeem Spence, awarding the free agent a three-year deal worth up to $10.5 million. In 56 games with the Tampa Bay Buccaneers, most serving in a backup role, he’s tallied 96 tackles and 5.5 sacks.

Those stats don’t exactly pop off the page, but it can be difficult to ascertain an interior lineman’s value simply by looking at the box score. Pro Football Focus, which evaluates every player on every play, loathed Spence’s performance last season, ranking him 123rd among 126 qualifying tackles. And that wasn’t an anomaly. They graded him similarly the previous three years. That’s just one assessment, but it’s troubling.

In addition to Spence, the Lions also brought in Jordan Hill and Ego Ferguson, a pair of former Day 2 draft picks.

In 2014, as a member of the Seattle Seahawks, Hill showed he can be a nuisance in the passing game, racking up an impressive 5.5 sacks and batting down four passes. But he’s struggled to have an impact beyond that year and is seeking to re-establish himself with an opportunity in Detroit.

Ferguson, a second-round pick in 2014, hasn’t appeared in a regular-season game since Oct. 2015 due to injuries. As a rookie, he recorded 23 tackles and two sacks in a reserve role.

Mixing it up with those young veterans will be rookie Jeremiah Ledbetter, a sixth-round pick out of Arkansas.

At 6-foot-3, 280 pounds, he’s built more like a 3-4 defensive end, but the Lions like Ledbetter’s fit inside in their scheme. He’ll likely have packed on a few more pounds before the start of the season, but his frame could give him some trouble as a run defender.

The positive is his high-end athleticism. At the combine, he recorded well-above average marks in the 40-yard dash and vertical and broad jumps, indicating good lower-body explosion. He also showcased adequate upper-body strength, putting the bar up 29 times on the bench press, despite long, 34-inch arms.

A junior college transfer, Ledbetter tallied 7.5 sacks and 15 tackles for loss in two seasons at Arkansas and offers intriguing potential as a pass-rushing interior lineman.

The path to make the roster for Hill, Ferguson and Ledbetter cleared up a bit when Khyri Thronton was suspended six games last month for violating the league’s substance abuse policy. He appeared in 13 games for the Lions last season, starting six, and could easily reclaim his job in Week 7.

jdrogers@detroitnews.com

Twitter: @justin_rogers

Lions camp battles

Day 1: Left tackle

Day 2: Nickelback

Day 3: Fourth, fifth receivers

Day 4: SAM linebacker

Day 5: Left guard

Day 6: Defensive ends

Day 7: Running back depth

Day 8: Backup defensive tackles

Wednesday: Backup quarterback

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