Kelly limped out of the New York Stadium clash just before half-time in what was only his second appearance since joining on a short-term contract a fortnight ago.
The 32-year-old again impressed at centre-back against the Robins, just as he did on his debut at Leeds, and his likely absence is a blow to the Millers.
Redfearn said that the former Republic of Ireland international picked up a strain, though the extent of the injury is yet to be revealed.
Whatever the length of Kelly's absence, though, Redfearn has been left frustrated as the injury comes hot on the heels of Leon Best - another impressive addition - having to miss three games through suspension.
"It’s a calf strain, which is a bit disappointing obviously because we’ve got Best in and he’s out for three games through suspension and then Kells gets an injury," the Millers boss said.
"I think just having them here gave us a lift during the international break, gave us a lift against Leeds and gave us a lift on Saturday.
"They’re proper players. They’ve come in and they’re the right types. They’ve given the place a lift."
If Kelly does not make Saturday's visit of Wolves, Redfearn could turn to Danny Collins once again in the central berth.
The 35-year-old was left out of the side at Elland Road - with the brand-new back four that day keeping a clean sheet - but he earned a recall at left-back against Bristol City.
And the veteran impressed Redfearn and will enter his thoughts ahead of the weekend.
"I thought Danny was good," he added. "I knew he’d played left-back at
Stoke before. It was important we got a big back four out because if you look
at their side it was the land of the giants.
"I knew that a lot of their play was
geared around set-plays. I thought Danny played it perfectly.
"Three
centre-halves and a full-back is no bad thing, particularly if your centre-half
is comfortable in possession, which obviously Danny is.
"He also gives you
another header at set-plays. And this division is about set-plays."
