The Pilgrims head to Alfreton Town (kick-off 7.45pm) in their first away fixture of the season and Knowles could feature after returning to full training following a calf injury.
His inclusion will be a boost to a squad which failed to muster many clear-cut openings in Saturday’s 1-1 draw against Chester FC, at the same time Alfreton were losing 1-0 at Brackley Town.
Karl Byrne and Tom Clare are also available for selection, having missed out on the opening day.
Luke Shiels and Tom Platt will come up against their former Alfreton colleagues for the first time. Alfreton, meanwhile, could include former United players Ryan Qualter and Shane Killock in their squad, although Killock did not feature on Saturday.
Last season's corresponding fixture ended 1-1 on New Year’s Day.
Tuesday’s match referee is Scott Tallis.
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