"We're still in talks and there are still three days left before the transfer window closes," Johnson told the club's official website.
"He's a player with Championship quality, a superb finisher and an experienced lad. We've spoken to Hearts and that's all okay.
"We've been speaking to Michal and now we have to convince him that Bristol City is the place for him.
"He's got some personal decisions to make. He's still got 18 months left on his Hearts contract and may decide to stay there.
"In my opinion, I think he will either come to us, or stay at Hearts."