Boyd has refused to rule out a return to Rangers after Ibrox manager Ally McCoist admitted he was planning a summer bid.
Johnston has yet to receive a call from McCoist spelling out if he will be offering the out-of-contract 30-year-old a deal but acknowledges Boyd has ambitions to play at the highest level he can.
However, the Killie manager hopes his team can offer the 19-goal frontman a suitable platform to show off his penalty-box talents.
He said: "I don't know what will happen with Kris. He's enjoying his football here. The club will try its best to keep him because he is such a massive player for us.
"It's all just speculation so there's no point talking to him about that. He is focused on the job ahead, which is getting points for Kilmarnock.
"I'm sure it won't just be Rangers who is looking at Boydy with the amount of goals he has scored. But we think he's happy here and will try to keep him for longer.
"I have not heard anything from Rangers. I would imagine they would contact us to tell us if they had been talking to him. But you never know.
"There's a lot to be said for enjoying your football, though. He's a Kilmarnock boy and I know he has loved his time here, so we can only try our best to convince him to stay.
"I know guys like Kris want to play at the highest level for as long as they can. We need to make sure we are challenging up the top end of the table if we want to keep hold of our best players."