Freeman’s book Lee’s Lieutenants is instructive. Lee grew short of officers. That was a huge element in the post-Gettysburg decline.
Lee grew short of capable officers. Corp, Division ,and Brigade officer slots were always filled. But the capability of those newly appointed officers was, in many instances, less than the officers they replaced.
He lost a lot of the good ones: Albert Sydney Johnston at Shiloh, Barnard Bee at Bull Run, Jackson at Chancellorsville, JEB Stuart at Yellow Tavern ...
Irreplaceable talent.