Mowatt, 18, made his first senior appearance for the club alongside another debutant, latest signing Scott Wootton, who headed Leeds' equaliser in the 3-1 victory little over a week since completing a permanent move from Manchester United.
"I gave Alex Mowatt his debut and he showed he can play a part this season because it won't phase him playing at home or away," McDermott said.
"Neil Redfearn has done a great job with the younger players and we should be proud of our academy."
Matt Smith also scored his first goal for the club, a controversial header to put his side back in front after former Leeds striker Billy Paynter had cancelled out Wootton's opener. Ross McCormack converted a decisive late penalty.
Doncaster boss Paul Dickov, who gave Smith his chance in professional football by signing him from non-league football when he was in charge at Oldham, felt the 6ft 6in forward had clearly fouled his marker when converting Lee Peltier's cross.
But McDermott added: "Whether Matt's goal was contentious or not I thought he challenged for the ball fairly and he's a strong lad in the air.
"In any case we had a complaint before they scored because Mowatt was fouled on the edge of the box before they gained possession."