Johnstone is highly rated by United chiefs.
“I enjoy it here. I love playing football here and training every day," Johnstone told the Aston Villa website.
“But the truth is I've not spoken to anyone about it as of yet.
“I want to keep playing football. I'm enjoying the number of games I'm playing.
“At the moment, though, it's very much a case of I've not spoken to anyone at United and we'll see in the summer.
“But I do want to carry on playing as much football as possible."