Sno, who signed for City on loan from Ajax last week, played the full 90 minutes in City's reserves game against Plymouth on Tuesday but Johnson does not believe the Dutch midfielder to be ready for an immediate first team starting role.
"I'm not saying he needs a lot of time, he just hasn't got 90 minutes of Championship football in him," the City boss told the club's official website.
"He's got some minutes in him and he's fighting for a place on the bench at Coventry.
"At the moment he hasn't had enough competitive games recently but it won't take him very long at all.
"He's not far away."