United are prepared to sell the 22-year-old, as Louis van Gaal believes he has enough cover with David De Gea and Sergio Romero already at Old Trafford.
Johnstone’s potential availability has alerted a host of Championship clubs, with six teams all chasing the England youth international.
If Johnstone did move to Wolves he would be in competition with the likes of Emiliano Martinez and Carl Ikeme for a place in between the sticks.
If the 22-year-old was to move to Loftus Road instead, his first objective would be to force Robert Green out of the team.
Johnstone has previously proven his class at League One level, and he helped Preston earn promotion from the third tier last season, during a loan spell at Deepdale.