The 22-year-old has been a regular part of the senior environment at Sunderland since arriving from Burnley 18 months ago, and has impressed when called upon.
But the arrival of Danny Batth has added further competition for places at the heart of defence.
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"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.”
Lee Johnson echoed Speakman’s sentiments and said he ‘would always be there’ for the centre-back.
"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."
Younger said the opportunity to challenge for league minutes was 'massive'.
“I’m delighted to be here," he said.
"It was a massive opportunity for me that I could not turn down.
“I want to be playing and this is the perfect opportunity for me to be around a first team environment and get some games under my belt.
“I am an aggressive and brave defender who loves to put their body on the line.
“I’m looking forward to what the future holds.”
On social media he added: "Thankyou to @SunderlandAFC for past year and a half or so, loved my time there met some great people and made some great memories, all the best for the future."
Younger is expected to be in contention for his Doncaster debut when they face League One promotion hopefuls MK Dons on Saturday.
Doncaster boss Gary McSheffrey said: “It was really important that we got a defender in, and when Ollie’s name was put to us we saw it as a good opportunity for the club.
“He’s got really good pedigree, he’s young and hungry which is the type of player we want here.
“He will be a good asset for the club and we feel he can be important for us in the short and long term.”