Teenage striker James Roberts scored a superb 84th-minute winner as Oxford came from behind to win 2-1 at 10-man Plymouth.Recalled Roberts - fit-again following a hamstring injury - turned three defenders before curling a measured 20-yard shot into the top corner from the right past former U's goalkeeper Luke McCormick. Oxford trailed 1-0 after 19 minutes but levelled when substitute John Campbell forced the ball over the line following a 70th-minute goalmouth scramble as Cheyenne Dunkley headed a free-kick back across goal. Plymouth turned early pressure into a 19th-minute lead when pacey striker Lewis Alessandra went through to score from midfielder Oliver Norburn's defence-splitting pass. Chased by United defenders Alessandra raced into the penalty area before rounding Oxford number one Ryan Clarke before slotting home from an angle. Plymouth were forced to reshuffle on 33 minutes when teenage central defender Aaron Bentley was red carded for a challenge on Oxford debutant Sam Long. In the second half Clarke did well to save Lee Cox's 20-yard shot, Bobby Reid sent an audacious 30-yard volley just wide of the post and Anthony O'Connor had his shot and follow-up cleared off the goal-line by a combination of Clarke and defender John Mullins.