It happens every year to pretty much every team.
GMs and coaches get a look at their roster in their first OTAs after the draft and weaknesses appear, someone isn't recovering from injury as quickly as you hoped or a player didn't develop as much as you thought, maybe some player came in 30 pounds overweight.
Whatever the reason, teams are almost always making some adjustments to the roster after OTAs. The Lions did it last year when they signed Isiah Buggs on July 23rd after the Lions decided they needed more beef upfront while John Cominsky was released by Atlanta on the 31st of May.
That's the other side of this, sometimes teams are pleased with the progress of a player or like the draft picks enough that they see an opportunity to shave a little cap space or just clear a path for someone else. Lions are sure glad Atlanta saw it that way.
That's two key players, Cominsky and Buggs that were picked up last summer, could Holmes find another player or two this year?

First, what positions might Holmes be looking to improve.
Okay everybody, all at once, they need a DT.
Hard to say they couldn't use one, Holmes promised some help for McNeil and Buggs and despite adding a 3rd round project and a rehabbing journeyman, he really hasn't come through on that promise, yet. But I have a name for you, Linval Joseph.
I know, he's old, he will turn 35 during the season and he only played 8 games last season, but they were the last 8 games after coming back from injury, and not only did start all 8 games. he started all 3 playoff games last year also. Philly isn't bringing him back with the way they are stacked at DT now. He played for 2 million last year and he would fit in great as the back-up NT and situational run stuffer so he wouldn't be overworked.
But here is why I find it interesting. The other day Glenn compared 3rd round pick Brodrick Martin to Joseph with those long arms and brute strength in that big body. Two things on that, why not have Joseph come in and teach Martin how to use those log arms, thereby speeding up the development of Martin. While the 2nd thing is, did Glenn have Joseph's name on his mind for a reason, has he been looking at tape of Joseph recently?

Next up, back-up QB. If they are being real about this being a redshirt year for Hooker, I don't see them sticking with Sudfeld as the back-up. This is another position Holmes said they were going to upgrade and if Hooker can't play, he hasn't come through on that promise either.
I'm going to say bring in Teddy Bridgewater and leave it at that, enough has been said already and we have more ground to cover.

I'm not going with WR, sorry. I know JaMo is suspended, but this is one of those times when the GM got a warm fuzzy by watching OTAs when Marvin Jones looked ready to fill in just fine, I can count on Marvin for 6 games and then fade into the depth chart when JaMo does come back, all while providing veteran leadership. You go out and get a Hopkins for example, it's great for the first 6 games, but then what. Now you are going back to more 3 WR sets when you spent the draft moving away from that with Gibbs and LaPorta, the two players that would be hurt the most by Hopkins being here. Nope, stay on course, you wanted more 2 TE and RB for a reason, and now I want to see it.

But I am going to add a sneaky need pick-up and that's RB.
Look, I know they already changed out the top 2 RBs with Gibbs and Montgomery for Swift and Williams. They also might have added their #3 RB with the UDFA Ibrahim, who I like. Still got Craig Reynolds and we hear Jefferson has looked good in camp.
However, that depth doesn't inspire me, our RBs become pretty pedestrian with any injury to Montgomery or Gibbs and we know how RBs go, there will be some missed time. I remember last year how limited the offense got when one of the big 2 was out and I want to avoid that this year if I can, and I can because of what is out there in Free Agency. Right now, I see one, possibly two RBs that would be perfect to add, Kareem Hunt or if that fails, the poor man's Kareem Hunt, Marlon Mack.
Hunt isn't that old, only 28 by the season, considering how little tread they took off his tires in Cleveland the last couple of years he should have plenty left in the tank. What makes him interesting besides his running skills is his receiving ability, if Gibbs is injured the whole offense doesn't have to change, plus Hunt is capable of filling in for Montgomery. Think about a backfield of Gibbs, Montgomery and Hunt. I kind of like that a lot.
It looks like there is some price adjusting going on with Hunt, not getting the offers he hoped for with all the RBs available this year from Free Agency to the draft, his market has been quiet. He made 6 mil last year and Spotrac gave him a market value of 7 mil, but I don't see that happening anymore. I would have no problem giving him 3 mil to come in and nail down that RB depth. Don't know if that's enough, but I like the idea of trying.

Lions are ready, their window is open and with these three moves, Teddy Bridgewater, Linval Joseph and Kareem Hunt, they get some veteran players with something left to bolster the depth and maybe even a little more. The only money this will really cost is Bridgewater, those QBs are never cheap, he played for 6.5 mil last year and would probably want around the same this year. Lions can do that and it could be the best money they ever spent. Another 3 mil for Hunt and 2 mil for Joseph, you don't even have to redo any contracts or make any salary cap related cuts to bring these three in and all three would have no Salary Cap impact long term either, one and done contracts most likely.
Veteran depth, that's the kind of thing championship teams do.