Mills was sent off against former club Doncaster on Saturday for deliberate handball and will serve a one-match suspension so Alex Pearce or Zurab Khizanishvili will start in his place.
The Royals held on for a 2-1 win at the Keepmoat to move to within a point of Ipswich and safety and need another victory over Plymouth to keep Paul Mariner's side below them in the drop zone.
Midfielder Gylfi Sigurdsson was absent on Saturday after taking a knock to his foot but he could return, though replacement Brian Howard scored one and made the second against Doncaster and should keep his place regardless.
Noel Hunt (knee), Chris Armstrong (knee) and Julian Kelly (shoulder) all remain sidelined.
Plymouth head coach Paul Mariner will be forced to change his starting line up for the trip.
Right-back Karl Duguid turned his ankle just before half-time in Saturday's 1-0 home defeat to West Brom and is ruled out, so his replacement, David McNamee, is set to keep his place.
Mariner handed a debut to new midfielder Damien Johnson alongside skipper Carl Fletcher, with youngster Luke Summerfield dropping to the bench.
He is set to stick with that pairing against the Royals, although new American striker Kenny Cooper could start in attack ahead of Rory Fallon.
Cooper was a second-half substitute against West Brom, but showed enough to suggest he could make a positive impact for Argyle.
Sporting Life