Johnstone enjoyed a successful spell on loan at Doncaster Rovers last season, making 18 appearances, and kept regular goalkeeper Ross Turnbull out of the side.
The 21-year-old's performances at Doncaster have not gone unnoticed with several clubs weighing up moves to take him on loan next season.
The likes of Fulham, Ipswich and Birmingham are all thought to be keeping tabs on Johnstone's situation as they look to bolster their goalkeeping options.
Johnstone is highly-regarded at Old Trafford and he was awarded a new three-and-a-half-year contract in January.
The talented custodian has been on the bench twice for the United's senior side and has represented England right through from the Under-16s through to the Under-20s.
United are thought to be willing to loan out Johnstone again next season so that he can gain some regular first-team experience as they are well stocked in the goalkeeping department and the Championship looks his most likely destination.