3. DL Christian Covington
This would go into the "well, we hardly knew ya" column. The Lions signed Covington in May, and added his experience (eight NFL seasons, 102 career games) to a defensive line mix that needed an injection of it. He's primarily a defensive tackle by trade and skill set, but he can play across the defensive line--and versatility is a trait the Lions' coaching staff covets.
A torn pec limited Covington to just four games with the Los Angeles Chargers last season, but a video he posted on social media early in the offseason showed he was fully recovered. So he lingered available into May on merit (or perception of merit/age), not due to an injury concern.
Even if a fairly notable addition is not made, the Lions will have an interesting training camp battle for spots on the interior defensive line. There are a few locks, and a couple potential wild cards, and how many defensive tackles will make the 53 heading into Week 1?
While his veteran leadership has inherent intangible value, as Christian Booher of SI.com pointed to, Covington will have to perform well during training camp to lock in a roster spot. If he's not trending toward making the roster, he may be let go so he can try to find an opportunity elsewhere.
Truly, it feels like Covington has no better than 50-50 chance to make the Lions' roster right now. Time will tell, perhaps quickly and early in training camp, if that feeling comes true.