Mark Atkinson praised the character and determination from his team to come from behind and win on Saturday.
Sunderland under-18s produced an impressive performance throughout at Rockcliffe, but fell behind when Middlesbrough turned home a free-kick midway through the second 45.
However, the Black Cats refused to down tools and dug deep, with Jake Hackett rifling home and Benji Kimpioka dispatching a stoppage time penalty to seal victory.
The result sees Sunderland remain unbeaten in group D in the Premier League Cup, leaving Atkinson delighted.
"The frustrating point of going behind was that it wasn't deserved," he said.
"In the first half we dominated and created plenty of chances.
"There was always that worry that if we didn't take our chances it might come back and bite us, which it did.
"The second half started a little bit slowly after a really positive first 45 and they scored against the run of play.
"But our reaction, character and quality was excellent," he continued.
"We scored two goals but we created other opportunities as well and scoring in the last minute was pleasing because that happened to us a lot last year."
