Johnstone has earned a recall after obtaining first-team football during a loan spell at Doncaster Rovers.
"Sam has been in our younger teams for quite a number of years now," Southgate told ManUtd.com. "At the beginning of the season, when we got together, he was injured for my first Under-21 camp and so, since then, he hadn't really been playing first-team football.
"He's done well at Doncaster and we felt it was a good time to bring him back into the fold really. He's got a lot of experience within the international set-up and knows most of the boys within the team.
"We've got good competition with him, Jonathan Bond, Jack Butland of course, and George Long, who is playing a lot at Sheffield United. When you've got a group of goalkeepers like that, they work well with each other but are also striving to get the shirt."
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