Had two meh-to-poor years. One good with a nagging injury, it was the most recent. Im not convinced either, but this is a big year for him. If he has a great season, panthers may be forced to use the tag on him. Plus with the rebound in cap space, he could easily get top 5 money on the open market if he has a great year too. The whole NFL is looking for speed and the whole league knows Djack has elite speed.
In a vacuum, competent O line depth is more valuable than a mid to late round draft pick. However, he's on a one year contract and the lions have pretty much announced they are doing a full rebuild. What benefit do you get keeping him in the first year of a complete rebuild? So that if Sewell gets injured, maybe you win 3 games instead of 2? Or maybe you think they should resign him to a big extension to be their 3rd OT, since that's what it'll take to outbid other teams that will gladly pay him to be their starter. Smartest move is to get something for him rather than have him leave for nothing in free agency next year and add to their draft arsenal as they build to the future.