Howard, expected to make his loan move from Barnsley permanent in January, has accumulated five yellow cards and sits out a one-game ban under the totting-up procedure.
Gary Speed could return to fill the gap. The 38-year-old has missed the last two games with a calf strain and remains a doubt, but Nick Montgomery and Greg Halford provide manager Kevin Blackwell with other options.
Matt Kilgallon is available again following his three-game ban, but Blackwell is not expected to split up the central defensive partnership of skipper Chris Morgan and Ugo Ehiogu.
Full-back Gary Naysmith is still nursing a groin problem, so Sun Jihai will continue in his place.
Plymouth manager Paul Sturrock is considering making changes when he takes his side to Sheffield for the second time in a week to face the Blades.
The Pilgrims claimed an impressive 1-0 win at the Blades' city rivals Sheffield Wednesday on Tuesday night but Sturrock is still set to shake up his squad for the trip.
The Pilgrims picked up a couple of knocks at Hillsborough, while Sturrock believes the long journey has also left one or two of his players feeling tired.
Winger Jamie Mackie and central midfielder Yoann Folly could come into contention after missing Tuesday's win with flu, while right-back Mat Doumbe (foot) and striker Emile Mpenza (knee) might also return.
However, goalkeepers Graham Stack and Lloyd Saxton are both sidelined with hand injuries, leaving Romain Larrieu as the only fit stopper at the club.
Sporting Life.