The Republic of Ireland forward, 28, has ended a loan spell at Ipswich Town early - after just one goal in his time in Suffolk - and has now agreed to join the Millers, to boost their forward options for the critical run to the end of the season.
Evans said: “We obviously needed to find a replacement for Alex Revell and Conor is someone I have liked for a long time.
“He has been unfortunate in the past couple of years and probably joined Ipswich on the view of Mick McCarthy telling him that (David) McGoldrick was going to Leicester.
“But he has played second fiddle to two of the best strikers in the Championship and last year, he was in competition with a loan striker (Chris Martin), with supporting players off him at Derby.
“He’s someone who first came to my attention when he was in the SPL at Kilmarnock when it was a proper league and scored a few goals.
“He;s got the right type of character we want. He’s big and strong and needs to add a few goals to his game, but he’s probably not played in a team consistently for a little while that make the chances we make. He gets that opportunity.”