Keogh has taken legal advice and is following a process supported by the PFA.
It is understood that the 33-year-old feels he has been treated unfairly by the club for which he has played more than 350 games.
He also feels he has been treated differently to Tom Lawrence and Mason Bennett – both of whom have been retained by Derby, despite their convictions for drink-driving.
Keogh was not charged with any criminal offence but was sacked by Derby
He was a back-seat passenger in the car driven by team-mate Lawrence that crashed into a lamppost after an ill-fated team bonding session that saw Lawrence and Bennett convicted of drink-driving.