Harley made his debut last Friday in the defeat to Bristol Rovers along with fellow loanee Guy Branston, who was red carded.
The 32 year-old will bring added experience to a squad in the spotlight, with the Millers having slipped to 13th place in League Two over the past six weeks.
“Everyone is feeling a bit under pressure. When you have a bad run of games like we have, you do feel it", Harley, who is with the club on loan from Notts County, told the Star.
“We have got to turn it around. Everyone has been in this situation before where they have had a bad run of results."
The Millers take on Shrewsbury on Saturday who have similar promotion aspirations, but have lost four away games already.
Harley is expected to continue in place of Tom Newey in the starting line up, whilst the suspended Branston starts the first of his three match ban.