The centre half switched from the north east to the Eco-Power Stadium on an 18-month deal after spending the last couple of years with the Black Cats.
Johnson insists he would have loved to have kept Younger as part of his squad but felt he would have been standing in the way of the 22-year-old’s development with his opportunities limited for Sunderland.
"I could have kept Ollie here, but to do that would have been selfish of me," he said.
"It wouldn't have been right for the football club, or for Ollie.
"I love Ollie, I love the way he trains, I think he brings a dynamic and an honesty to training, and I really wish him the best of luck.
"I'll always follow him and I'll always be there for him if he ever needs anything.
"It's great for him to kick-start his career in League One at a really good club where he will be able to compete straightaway for a place.
“I'm sad to see him go because players like that, I have a huge amount of admiration for on a personal level."
Sunderland sporting director Kristjaan Speakman also offered his thoughts on Younger's departure.
"Ollie has really progressed over the last 18 months, but not populated the starting eleven enough due to the competition between our senior players," Speakman said.
:We believe that it’s important he continues his development to build a career and Doncaster represents a fantastic opportunity for him.
"Whilst the selfish approach would be to retain him, we need to support our young players with the right decisions at the right time.
"He’s a great kid and we wish him well.”