The former Spurs and Reading full-back, 32, has made an excellent impression since joining the Millers last month on a deal for the rest of the current campaign.
But in just his second game for the club last weekend - in the 3-0 win over Bristol City - Kelly was forced off with an unfortunate calf injury just before half-time when he was replaced by Farrend Rawson.
The one-time Republic of Ireland international was Redfearn’s second permanent capture following the signing of forward Leon Best, having been without a club since leaving Reading at the end of last season.
Kelly made his debut in the Millers 1-0 win at Leeds United, with Redfearn praising his outstanding performance.
It had been feared that he could be out for several weeks, but Millers chief Redfearn has allayed those concerns.
Redfearn, whose in-form side welcome Wolves on Saturday, said: “It’s not as bad as first feared.
“It’s a calf strain, but a grade one, so it’s days rather than weeks, which is good news because Stephen is obviously a big influence on what has happened recently.
“It’s a shame to lose him - and whether he will be fit for Saturday, we don’t know - but he’s been a massive signing for us.”