Gen. Robert Mitchell, Chief of Cavalry, Asks to be Relieved, Oct. 1863

General Robert Mitchell, Chief of Cavalry in place of David S. Stanley (on sick leave), also becomes ill and asks to be Relieved of Command

Army of the Cumberland

General David S. Stanley, Chief of Cavalry, grew ill and asked to be relieved of duty in September 1863. He then went into hospital at Stevenson, Alabama. General Robert B. Mitchell then took command of cavalry in September 1863.

General Robert B. Mitchell is in ill health; wants to be relieved of command of Chief of Cavalry (October 18, 1863).

Mitchell to Rosecrans, Request to be Relieved of Command 
(Official Records, via Google Books)

Official Records, Series 1, Volume 30, Part 4, page 462:

[QUOTE]:
Headquarters Chief of Cavalry,
Decherd, October 18, 1863.

Maj. Gen. W. S. Rosecrans,
  Commanding Department of the Cumberland:

  General: I have received no dispatch from you since I arrived here, although I have sent you several. Have you not received them? I am doing all in my power to put the command again in good condtion, and am getting rations, clothing, horseshoes, &c., as fast as possible; but it is positively necessary that we get a great many new horses before the command can be of much service--the stock was so badly used up in the pursuit of Wheeler and Roddey. As soon as possible I wish to be relieved from duty in the cavalry command, as I cannot, if I have any regard for my health, continue longer in it. The severe service devolving on me since having been on duty in this arm of the service has rendered the state of my health much worse than formerly, and the chances of my ultimate recovery more remote, and I feel that in justice to myself I should not expose myself any longer as I have been obliged to do for the last four months. When I last saw General R. S. Granger he expressed a desire and seemed very anxious to be assigned to the cavalry command. I should like to see him assigned to that duty. I will not leave anything undone till I get the command in condition.
  I am, very respectfully, your obedient servant,
Robt. B. Mitchell,
     Brigadier-General and Chief of Cavalry
[END QUOTE]
[page 462]

Official Records, Series 1, Volume 30, Part 4, page 462
Short URL: http://bit.ly/OR-s1-vol30-pt4-p462-Mitchell-to-Rosecrans 
PAGE 462 VIA GOOGLE BOOKS
_________________________________________

David Sloane Stanley

According to a historic source, as presented on the 1st Battalion 22nd Infantry website (under David Sloane Stanley: Commanding Officer 22nd Infantry, 1866-1884), Stanley was on sick leave from September to October 1863, and then returned to his command. He was appointed Major of the 5th Cavalry on December 1, 1863. Source (see doc. image of Cullum's Register, entry on David S. Stanley):

David Sloane Stanley (web page).
http://1-22infantry.org/commanders/stanleypers.htm

The doc. image shows the military history of David Sloane Stanley (in Cullum's Registers, 1868-1901). This looks like page or section 1544 from the historic source.

General Stanley was at Cleveland, Tennessee, not long before the troops were to march from Cleveland with the goal of advancing on Atlanta. Here are two sources that put him at Cleveland, Tennessee, around the beginning of April 1864:

The first one is from page 93 of Tennessee Civil War Sourcebook by the Tennessee Historical Commission (edited by James B. Jones, Jr.), for the month of April 1864 (published on Art Circle Library website, a service of Art Circle Public Library, Cumberland County, Tennessee):

"LOUDON, April 2, 1864.
Maj.-Gen. SCHOFIELD:
Gen. Stanley reports that a large force rebel cavalry was seen 8 miles east of
Cleveland this morning at sunrise moving in the direction of Charleston. The
commanding officer at that post is on the alert.
G. GRANGER, Maj.-Gen."

Tennessee Civil War Sourcebook, April 1864 (PDF, 128 pages, one chapter of "book")
http://www.artcirclelibrary.info/Reference/civilwar/1864-04.pdf

The second one is Lt. Louis A. Simmons, author of The History of the 84th Reg't. Ill. Vols., who says that General Stanley, in person, made a very detailed inspection of troops in April 1864 at Cleveland, Tennessee (161-162). I am a bit puzzled here. I know that the new cavalry chief, James H. Wilson, made a close inspection of cavalry troops at Nashville and other places; but maybe he sent down orders to have other cavalry commanders do the same and report back to him.

History of the 84th Reg't. Ill. Vols.
https://archive.org/stream/historyof84threg00simmo#page/160/mode/2up

Related Posts (this blog):
Robert B. Mitchell, David S. Stanley, Army of the Cumberland, Organization of Army

Muse's link to Tennessee Civil War Sourcebook (and biblio info)
http://musescivilwar.blogspot.com/2016/09/tennessee-civil-war-sourcebook-links.html