Left-back Ridehalgh has arrived on an emergency loan from Huddersfield while centre-back O'Connell has joined on a youth loan from Blackburn and, with a crippling injury list, Steve Evans is likely to throw them straight into his starting XI.
Evans also recalled centre-back Johnny Mullins back from his loan spell at Oxford and he is likely to get an outing as the Millers boss contends with a host of absentees.
Lee Frecklington (back), Lionel Ainsworth, Kari Arnason (both shoulder), Dale Tonge (ankle/knee), Scott Shearer (knee), Kieran Agard (thigh), Craig Morgan (leg) and David Noble (hamstring) are all out.
A knee injury to left-back Josh Morris cut his loan spell from Blackburn short, prompting Ridehalgh's move, while Jamie Devitt returned to Hull last week with a hamstring strain.
The good news for Evans is that midfielder Michael O'Connor has served a one-match ban and striker Alex Revell has recovered from the illness which forced him off in the second half of Tuesday's abject defeat to Wycombe.