Johnson expects the winger will complete a move to Bramall Lane in the "next couple of days", but Blades manager Kevin Blackwell would not be drawn on the matter.
Johnson told City's official website: "Macca is talking to Sheffield United, he hasn't been training with us.
"I expect that deal will go through in the next couple of days, but that's all I can say at this stage."
Scottish winger McIndoe, 29, arrived at Ashton Gate on a free transfer from Wolves in August 2007 and has made over 90 first-team appearances for the Robins.
Johnson also confirmed he was close to completing a deal to bring an unnamed player in the opposite direction, adding: "We've agreed terms with him and he now has to decide whether he wants to come here, or take a lucrative deal from abroad."