Johnstone has joined Scunthorpe United on loan until 8 October.
"I am excited and happy to be here, it's going to be a good experience for me and I can't wait to get going," Johnstone told Scunthorpe's official website.
Scunthorpe manager Alan Knill already has Reds midfielder Oliver Norwood on his books, with the Northern Ireland international making an eye-catching start to his temporary stint at Glanford Park.
"Oliver told me before that Scunthorpe had asked about me," revealed Johnstone, "but I only found out on Wednesday morning from Sir Alex Ferguson and United's goalkeeping coach Eric Steele.
"I spoke to the gaffer and he said 'go and enjoy it, it will be a good experience for you'."