Johnstone has featured in 11 matches for the Yorkshire club and has been getting rave reviews from the coaching staff at the Keepmoat Stadium.
"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 retain his place as Rovers host Birmingham on Saturday.