Sure it is.
Offense has always ruled baseball. Owners figured out over a century ago that fans don’t pay to watch players field. If you can hit, you’re more likely to make the majors than if you can field.
But fielding is a unique skill set, and not one everyone has. A lot of fans think it’s easy or easily learned, which is not necessarily true. Even on this thread, someone said something about “the Sox don’t have infield practice anymore,” as if all it took to becoming a great fielder was repetitive practice drills.
Ted Williams once said the hardest thing in sports is hitting a round ball with a round bat squarely. Yet Williams mastered hitting like no other, but was fairly questionable as an outfielder. You’d think he’d call the activity he found more challenging to be the difficult one.
But he didn’t. And while hitting is tough, so is doing the complex math in your head to extrapolate the curved path of s flyball until it intersects with the plane of the outfielder. As bizarre as that sounds, that’s what it takes to catch a fly ball. And when we say an outfielder “takes bad routes” or “gets bad jumps”, what we mean is he cannot do that math in his head fast enough.
It’s not just simple coaching and teaching. Just like hitting, there has to be a foundation to work with.
Is it possible the Sox don’t coach defense well? Sure. But the evidence has to be diverting other than “name one outstanding infielder since Betts.”
By the way, Betts learned to become an elite outfielder really fast. Don’t you think the Sox developmental coaches had a hand in that?