Alex Fisher needed 25 stitches in a cut lip, sustained in the 1-0 defeat by Colchester in the quarter-finals of the Football League Youth Alliance Cup at Milton on Saturday.
Oxford, who had four under 16s on the pitch at the end, were unlucky to lose the game.
Not only did they feel they should have had a penalty in the incident in which Fisher was injured, but Declan Benjamin hit a post and they went close on several other occasions, but couldn't convert their chances.
Colchester's winner came from a 60th-minute shot.
Said coach Mickey Lewis: "The turning point was when the ball was played into their box and Alex Fisher got to the ball just before the keeper who punched him as he headed the ball.
"Anywhere else on the pitch and it's a foul, but the ref only gave a goalkick.
"Alex had to go off to have the cut sewn up and had 25 stitches. Our thanks goes to the staff at the John Radcliffe for their help."
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