Interesting Chronology for this picture:
Buell became a Brig. Gen. on May 17, 1861 and left the AOP and reported West on November 1861.
William Farrar "Baldy" Smith became a Brig. Gen. on August 13, 1861. Left the AOP March 4, 1863.
Silas Casey became a Brig. Gen on August 31, 1861
This can probably place this picture between early September and late October 1861.
Yes, Casey was the only one that was left with the AOP until the end of the war. And other than Casey, McCall and Blenker, certainly that was not a squad that their replacements felt that they were All-Star level...