The centre-back, an ever-present since arriving at Home Park in January, had to be substituted in Saturday's 2-1 defeat at Gillingham with a twinge in his groin.
And Charles, who celebrated his 32nd birthday on Monday, will be assessed by the club's medical staff ahead of the first of two back-to-back home games for the side rock-bottom of the Football League.
Striker Gozie Ugwu shrugged off an abdominal muscle injury to feature as a second-half substitute and the Reading loanee is set to be involved again.
Goalkeeper Rene Gilmartin (hamstring), right-back Durrell Berry (knee) and midfielder Jamie Lowry (ankle) have all returned to training but this game may come too soon for the trio.
Meanwhile captain Conor Hourihane was booked for the ninth time this season at Gillingham and a yellow against Bradford would see his miss the Pilgrims' matches against play-off hopefuls Fleetwood and Southend.
- Sports & Recreation