The 21-year-old was originally recruited by Lee Clark on January transfer deadline day but was stretchered off 14 minutes into his debut against Derby County with an ankle injury.
Thorpe returned to his parent club for treatment before confirmation of an extended spell on the sidelines resulted in the England U21 international being formally recalled by the Red Devils at the start of March.
However, now back to full fitness, Blues boss Clark has once again swooped for the defender who has captained England at Under-19 level and skippered United's youth side when they won the 2011 FA Youth Cup.
Thorpe also helped United to victory in the U21 Premier League trophy last year against Tottenham Hotspur.
Clark has had United's Jesse Lingard on loan earlier in the campaign and currently has Federico Macheda and Tyler Blackett in the St Andrew's ranks.
Thorpe will be in contention to feature against Bournemouth on Saturday.