The 24-year-old goalkeeper is spending the season with the Championship club, who hope to snap him up on a permanent deal.
Taylor has indicated that Johnstone's deal has already been extended.
"I'd like to see him here permanently — any player playing as well as that you would like to keep," Taylor said.
"But clubs like United don't extend your deal for no good reason.
"Sam can be as good as he wants — he's still a young goalkeeper.
"He should kick on from here and the sky's the limit for Sam.
"But the most important thing is he's playing football at a decent level — he's not sitting on the bench in the Premier League."