"Marnick's just got a little bit of feeling that he had from the first-team game the other night [against Newcastle]," explained Joyce. "To be fair to Marnick, he played in the first team on very little training really so it was a credit to him to be able do that.
"He was still suffering a little bit from it and we can't afford to risk it when he's not quite sure and he could get something and be out for a long time."
Warren Joyce's table-topping Under-21s are next in action on Monday night with a home game against second-placed Southampton at Moss Lane.
More: Sunderland U21 0 United U21 0 | United U21 4 Newcastle U21 2