QB - I honestly can't say which (if any) of the QB prospects available in this draft - at any pick - might be viewed by BB as an adequate (or better) candidate to try to develop into Brady's replacement. I really have no idea how BB might evaluate such prospects, and I absolutely do not trust any QB scouting reports from anyone to be any closer to how BB actually sees things than I am. So, I can't project that BB won't sell most of the Pats 2018 and 2019 top draft picks draft to move up into the top-5 for a QB prospect, and I'm equally unable to project that he won't just skip QB this draft altogether. AFAIK, there are roughly equal chances that it could go to either extreme or end up somewhere in the "middle." A drafted QB would seem unlikely to contribute much in 2018, regardless when he's selected.
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LT - Substitute "Scar" for "BB", and "LT" for "QB" in the above.
OL - However, in terms of depth along the OL, looking ahead to 2019 - when Mason becomes a potentially unaffordable UFA, Waddle becomes a UFA, and Marcus Cannon will be 31 - there doesn't appear to be a lot of it (depth, that is). Acquiring more prospects in this draft for Scar's pipeline seems highly likely to me. That could happen in almost any round, but such guys would be unlikely immediate contributors nevertheless.
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SAFETY - A prospect who the Pats have graded as a "day-one starter" could almost immediately become a bigger contributor to the Safety rotation than Richards (even with everyone healthy), and provide significant high-quality snaps if one of Harmon, Chung or D-Mac gets injured. He'd be more a "#2b/#3b" than a mere replacement for Richards at #4. Having a 4th "starter-capable" safety also significantly lessens the need to find a faster (lighter) "coverage LB" who's also consistently stout against the run. [And, if the defense is going to start relying on a specialty "coverage LB", the Pats had better get two of them. Otherwise, if/when the first guy gets injured, they're back to relying on Richards.]
... There may well be at least a couple prospects in this safety class that that Pats have graded as "day one starter" who could be available in the #23-#43 range, but very possibly no later than that. Since it happens so rarely that potential "day-one starter" safeties fall anywhere near the Pats typical pick range, it seems at least somewhat likely that the Pats could take this opportunity to select one.
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LB - Although the Pats often use only two LBs (and rarely more than three), they have only Hightower and Van Noy as "proven starters", especially for ILB/MLB assignments. A "day-one starter" prospect could immediately make significant contributions as the third LB, and as a high-use rotational player for 2-LB sets. A second "potential starter" grade draftee could become an immediate upgrade over one of the current reserves, and also see a decent snap count in rotation.
... It seems like there may be as many as 3 or 4 "day-one starter" level prospects available in the #23-#63 range - perhaps even a couple more if you include OLB/edge types. It doesn't seem to me to be outside the realm of possibility that the Pats double-dip here - unless they're pretty certain that one or more of Rivers (IR), Langi (IR), Roberts and M.Flowers is likely to be the equivalent of any 2018 prospect who's likely to be available to them when they pick.
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"DE" - Since, at this moment, I'm not seeing a pressing need for a 3-4 type DE (6'4"+, ~290-300 lbs), I'll assume that we're discussing primarily 4-3DE type prospects (6'2"+, 260-280 lbs), rather than 3-4DE types or even OLB/edge types. [BB may see things much differently, though.]
... In 2016, when the Pat had Nink, Flowers, Sheard and Long on the roster at 4-3DE, those four ended up averaging nearly equal snaps per game in different pairings (bracketing Branch and Brown in the middle). All four were essentially "starters", and that turned out to be a very effective and resilient approach. Currently, the Pats have Wise, Clayborn and Flowers (who's a UFA at the end of the season, and who may be unaffordable). Behind them, they have Lee (who at least showed some promise in his pseudo-rookie season), and Davis (untested, coming off INJ) ... and Hightower, possibly. A "day-one starter" prospect could make substantial contributions in a regular four-man rotation, and be ready for a larger role in 2019 if Flowers walks.
... At least a few "day-one starter" types - for the way the Pats use them in their scheme - will likely still be available at #23 and a couple might still be available as late as #95.
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CB - For 2018, after Gilmore and J-Mac, evaluations seem a tad less certain for the next two on the depth chart (Rowe, JJ). After them, things get really shaky (Cy Jones, Lewis). For 2019, the situation may become dire, since only Gilmore and Cy Jones are under contract beyond the end of the 2018 season. So, selecting a 2018 CB prospect who's graded anywhere from "3" (developmental) and "1" (day-one starter) may be appropriate.
... While CB draft classes are typically much deeper than Safety draft classes, CB draft classes often turn out to be not quite as deep as they're proclaimed to be pre-draft. So, if the Pats coaches deem selecting a "day-one starter" type as the most appropriate, that may need to happen by #63.
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TE - While there certainly appears to be an opening for a #2 in-line TE ("day one starter") on the roster, I'm, not at all certain that there's a prospect in this draft who is capable of filling it.
... As BB himself has said, TE may be the most difficult position to learn in the Pats offense. A TE needs to understand and execute nearly everything that and O-lineman does, AND to understand and execute nearly everything a WR does. So, a TE prospect who has shown little or no aptitude for blocking is probably not even on the Pats board, regardless how well he may run routes and catch. Then, the question becomes, "Is there a TE prospect in this draft who has demonstrated sufficiently high aptitude at both blocking and receiving ("day-one starter" grade) that he'd represent more immediate value to the team than the safety, DE, LB, OL or QB that they might need to forego in order to select him in the 1st or 2nd round?" If so, that TE prospects contributions on offense could be substantial almost immediately. If not, they may add a prospect in the 6th or 7th or as a UDFA as OTA/Camp competition for Allen, Niklas, Hollister and Tye.
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RB - Since BB appears to favor an RBBC that features more more less interchangeable Swiss Army Knife type players in a regular rotation who can all run, catch and pass-protect fairly well, I'm not sure how the "day one starter" designation would actually apply here. In terms of being a contributor/"potential starter" (IOW, one of the three gameday actives), a prospect would need to be able to grasp the blocking schemes and the passing system quickly, and outperform both Gillislee and Hill in OTAs and Camp, to see significant playing time. The prospect would need to surpass at least one of Hill/Gillislee in order to even make the roster.
... It seems to me that's very nearly as likely to happen with a 4th or 6th round prospect as it would be with an RB prospect taken in the 1st or 2nd, so I don't expect one of the picks from #23 thru #63 to be used on an RB.
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WR - A young prospect could have some value as added competition for the "old favorites" who are already on the summer WR menu, but the "need" to spend a 1st or 2nd round pick on one (at the expense of a prospect for some other position) seems to be approximately zero from my perspective.
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CHEERLEADER - Has anyone considered drafting a cheerleader with one of the top four picks yet? Have there been any scouting reports posted? Or, more importantly, "highlight" tapes? Should we start a thread for that?