The Black Cats boss named just one 'qualifying' first-team player by the competition’s rules as Denver Hume captained the side.
Hume has just recovered from a hamstring injury and needs game time, yet Johnson resisted the temptation to play any other senior players.
Sunderland’s youngsters made a positive impression, though, as Tyrese Dyce put his side ahead in the second half at the Stadium of Light.
United drew level after an excellent strike from Zidane Iqbel, yet Stephen Wearne’s header secured the win for the hosts.
Following last week’s 2-1 win at Lincoln, Sunderland have qualified from Northern Group F with a game to spare, against League Two side Bradford.
Our chief SAFC writer Phil Smith has delivered his verdict on how each player fared against United’s under-21s side.
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1. Jacob Carney - 7
Little he could do about Iqbal’s stunning strike but his shot-stopping otherwise was very impressive and he showed his reflexes on debut. Distribution settled after a fairly shaky start. 7 Photo: FRANK REID 2021
2. Kenton Richardson - 6
Rarely beaten defensively and got forward when he could. Another steady performance. 6 Photo: Frank Reid
3. Patrick Almond - 6
Given a tough test by Joe Hugill throughout but came through it and played a big part in the winning goal with a fine drive from the back and pass out to the wing. 6 Photo: FRANK REID 2021
4. Oliver Younger - 8
Again looked a cut above at the heart of defence. Really strong in the challenge and used the ball well. Has had a great tournament so far. 8 Photo: JPI Media