Johnstone joined Preston North End last January on loan and returned in May having helped the team gain promotion to the Championship.
The 22-year-old impressed during United’s preseason tour and played more minutes than any other player.
However, the Englishman is keen to move if he is not going to get any game time this season under Louis van Gaal.
United currently have five goalkeepers on their squad list who could realistically feature next year but three of them – David de Gea, Victor Valdes and Anders Lindegaard – have been touted to move away before the end of the transfer window. If Johnstone was to move, that would make four and would leave only Sergio Romero at the club and a lot of pressure on his shoulders.
For all of United’s spending this summer in key areas, it could all come undone if De Gea does not stay or a suitable replacement for the Spanish international is not found.