Thorpe, who joined Blues on the final day of the January 2014 transfer window, was originally scheduled to remain at St. Andrew's until the end of this season.
However, he lasted just 14 minutes into his Blues debut against Derby County, before being stretchered off with an ankle ligament injury.
He subsequently returned to United, where he is still receiving treatment at their Carrington training ground.
At the time of his injury, Blues boss Lee Clark said of Thorpe: "He's a terrific kid and we're devastated for him.
"We were absolutely delighted to get someone of his quality. Manchester United rate him so highly and he's been a brilliant lad."