The Scottish striker has scored 19 goals for Killie this season in the Scottish Premiership, which is nearly half of their overall tally.
"It can't just be Kris scoring all the goals," Johnston is quoted as saying by The Scotsman. "Rory McKenzie has done that for us a little bit this season, doing well from that wide role out on the right, using his pace to get in behind teams.
"He is a real threat when he plays like that and you saw that with his goal against Motherwell last Saturday. But we can't rely on just one player like Kris to score the majority of our goals. It's great that we have got someone of his quality.
"However, we need everyone to chip in. We do have players capable of scoring goals if they get into the right areas."
Kilmarnock are currently sitting in eighth place in the league table.