I had this in one of the threads in my forum, but got too many questions on twitter so wanted to post it here so I could link to it... then I decided to edit it a bit so it would make sense as a stand alone entry.
Lions started Free Agency with $31.4 mill in cap space (all numbers used are subject to change or may be rounded slightly up or down).
Tendered OT Lucas -$1.797 million in cap (this cap used is not reflected in NFLPA numbers as of yet, it hasn't been signed by Lucas).
Signed DT Kyri Thornton -$1.5 mill in cap
Signed OT Rick Wagner -$5.9 mill in cap space
Signed DT Akeem Spence -$2.25 mill
Signed DE Cornelius Washington -$2.3375 mill
Signed CB DJ Haydan -$3.75 mill
Signed WR Keyshawn Martin -.775 mill (not sure all of this is in 2017 yet, depends on bonus structure if any)
Signed G TJ Lang -$5.916667 mill (but $46,875 of it is NLTBE and would come off carryover into 2018 if he plays all 16 games)
Signed LB Paul Worrilow -$3 mill (and another NLTBE bonus of $1 mill... others have these NLTBE bonuses too, possibly team goal oriented).
Signed TE Fells -$1.5 mill (a guess, not sure on bonus details).
10 contracts fall off (top 51 rules apply) +$4.6 mill in cap added (rounded down, so its more then this)
Leaves Lions about $7.32 mill in salary cap space as of today. On June 1st Levy's cap is freed up, adding about $5.99 million.
They'll need about $2 mill to sign their draft picks (with top 51 rules, depending on trades of picks).
So they have about $5 mill to play with now... and will need or at least want Levy's near 6 mill for the season.
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All that said, they could cut or trade a player or two to free up space... and/or they could do one or more extensions. For example, Stafford has a salary of about $17 million in 2017... if he were to get a 4 year extension they could reduce the 2017 salary down increasing his signing bonus by that much, and spread that one year's salary over 5 cap years. It's not great cap management but truth is, he's going to get that money anyway, and it's only one year, and it's the last year of his contract - it's not like there are multiple cap pushes mounted on top of each other to create a horrible future cap situation.
Other players with potential roster spots that might see action of a sort is Ngata and Whitehead amongst others.
I'm not advocating the Lions do any of the above, just suggesting that they are not locked into their current cap situation, they have options.
