The Pats could send their 2019 1st round pick, and Detroit's 3rd round pick for Mack.
They would have to sign him to at least a five year deal with a large signing bonus and a decent amount of guaranteed money over the first three years, but if they did that, they can make his cap hits in the 15 million a year range, which is manageable. They would have to come up with at least as much guaranteed money as he would have gotten if he did nothing (13 mil this year, and then the 17+ mil franchise tag (most likely twice) so say @ 50 million. Mack is 27, a year or so older than most guys when they come to their first big FA period. So he may be willing to sign a 5 year deal, provided that the last few years are not "funny money".
Right now the Pats have 9.3 million in cap space, they can free up another 10 million in cap space if they cut/trade Allen, J. McCourtey, and Gillislee.
Does anyone here think that if you inserted Mack into that pathetic defense that got smoked by a back up QB in the Superbowl, he wouldn't have made enough plays to make a difference?