Patriots Defensive Woes

My wife is an diehard Patriots fan. What we both cannot understand is why they will not get a defensive coordinator. Now I know Bill Belichick is an excellent coach, but a great mind can always use another excellent mind to work with. The great Patriot teams of the earlier decade had far less athletic defenders. But they also had great defensive coordinators to take up the slack. When I look at their current defense I see potential greatness with great young talent. Talent that proved themselves at the college level. Now before anyone starts, this team needs fundamental work, missed tackles, not turning around looking for the ball (leads to pass interference), and blown assignments, etc. Anyone else feel the same about them. The last three years of sorry defense. By the way I have three favorite pro teams. Ravens and Jaguars (two long stories of how they became my teams) and where my heart is always at The Rams (proof that coaching does help, these team couldn't tackle a kitten the last six or seven years now they are starting to turn the corner on defense). Proof that fundamental fixes can greatly improve a defense. Any comments.