Based on the fact that he received advance commutation pay for sick furlough October 6, 1863, I would assume that he's been hospitalized for much of the previous month missing Chickamauga.

John Harrell, Private, Company I, 22nd Alabama Infantry, received on October 6, 1863 at Montgomery, Ga. $9.90 for commutation of rations while on sick furlough from Oct. 6 to Nov. 4, 30 days @ 33 cts., no other records