But he has yet to be convinced that the man fronting a Singapore-based consortium bidding for control of the Glasgow giants is fully aware of the task he could be taking on.
Johnston - who was opposed to Craig Whyte's takeover last May - told the Scottish Sun: "I've done my own checking using my sources in America and Singapore.
"I'm quite familiar with the guy (Ng) and what he's doing. I just want to make sure in my own mind that his primary purpose is to serve the interests of Rangers Football Club.
"As I understand it, the man is credible, he is genuine. He has some money and some monied partners.
"It's whether I really think he knows what he's getting into. Does he understand the expectations of Rangers fans? The jury's still out as far as I'm concerned.
"The money also has to be real and available very quickly. If the new owner does not have the ability or the willingness to put money in very quickly that's going to be a huge disappointment, possibly even a barrier to the continued support of the club."
Former Ibrox director Paul Murray's Blue Knights took a step back from the bidding process on Monday night but are not necessarily out of the running completely.
American tycoon Bill Miller is the other interested party.
Sale Sharks owner Brian Kennedy - who previously had an indicative offer rejected for being too low - indicated on Tuesday that he could reconsider his position if he feels Rangers' future is in jeopardy.