The 21-year-old joined Donny on loan from the Old Trafford club on January 31.
"He's had a good bit of experience out on loan and the feedback has been really good from Sam as well," United reserves coach Warren Joyce told his club's official website.
"It's a good test and a good challenge for him. He looks like a commanding figure at Championship level."
Johnstone is likely to make his 12th first-team appearance for Doncaster during Saturday's league clash with Birmingham City.