Arnason has missed the last two games through suspension and will return to Steve Evans’ starting line-up against the Seagulls.
Striker Danny Ward is also likely to return after a five-match absence with foot and knee problems and he too could be handed a start.
Evans may also bring in Jordan Bowery as he tries to arrest a four-match losing streak which has seen the Millers slip to just four points above the drop zone.
Brighton, five points ahead of the hosts, will be without captain Gordon Greer through suspension.
The Scotland centre-half starts a two-match ban after collecting his 10th booking of the season during the 1-0 defeat by Norwich, so Lewis Dunk will return to the back four alongside Greg Halford.
Midfielder Mustapha Carayol is awaiting his debut having been an unused substitute on Friday following his loan switch from Middlesbrough.
Sam Baldock, Andrew Crofts (both knee), Kemy Agustien (thigh) and Solly March (calf) remain on the sidelines.