Here's how I see the current CB group ...
.. Gilmore - very good, relatively young, under contract thru 2021
.. Jason McCourty - likely to be solid, but probably not a long term contributor (turns 31 in August)
.. Jon Jones - more than merely a serviceable backup/ST-guy, was statistically slightly better than even Butler last season (2018 RFA)
.. Rowe - could be good, if he could stay healthy; UFA at the end of the season
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.. Cy Jones - low confidence due to his lack of performance/contributions before his injury
.. Ryan Lewis - highly touted, but untested in live fire
.. Jomal Wiltz - a complete cipher
-- While Jon Jones appears to have the potential to be very good, it's still uncertain (except perhaps to the coaches).
-- If Rowe continues to have trouble staying healthy, he may not be renewed for 2019 (and might be good for a 2020 comp pick).
-- Given Jason's age, it seems unlikely that he'll continue past 2019.
I definitely think there's room/need for a developmental prospect - not necessarily a guy from one of the Pats current first 2-3 picks, but possibly more than a UDFA.
This possible roster opening is among the reasons I think that a trade down for more picks is more likely than a trade up for one "special" prospect this year.