The reliable centre-back, who captained the Reds to FA Youth Cup glory in 2011, was hurt on his debut for the Midlanders in the draw with Derby County last month and taken off on a stretcher.
"It's frustrating for the boy," Reserves coach Warren Joyce told ManUtd.com. "I did say to him that I've never known him be injured or even miss a day's training due to illness or anything else.
"It was a really good 14 minutes he had on his debut as well! I watched the game and was pleased with him.
"Every touch he had was impressive. He made important headers under his own bar, defended corners and started really well. It looked like he had been playing there for 10 years and not 10 minutes."
It was the first loan move of Thorpe's career and the 21-year-old will be determined to regain full fitness under the supervision of the United staff at the Aon Training Complex.