Long-time Trump Organisation CFO Allen Weisselberg has been subpoenaed, the Wall Street Journal reports. That should come as little surprise; he was invoked twice in Michael Cohen's taped conversation with President Donald Trump about buying the rights to former Playboy model Karen McDougal's story.

"I've spoken to Allen about how to set the whole thing up," Mr Cohen says on the September 2016 recording. He adds: "I spoke to Allen about it, when it comes time for the financing. ..."

But that doesn't mean it's not a potentially huge moment. More than anything - more than whether Mr Trump proposed paying cash for Ms McDougal's story or warned against it - this could be what matters on that tape.

Bloomberg's Timothy O'Brien spotlighted the potential significance of Mr Weisselberg's implication in the whole thing shortly after the Cohen-Trump tape came out on Tuesday evening. Mr Cohen, Mr O'Brien has argued previously, is a small fish in Trumpworld; Mr Weisselberg, by contrast, is deeply involved in Mr Trump's business and finances:

Mr O'Brien said: "Weisselberg . . . has worked for the Trump family since the 1970s, and knows more about the Trump Organisation's history and finances than nearly anyone. Almost 71 years old, he joined the company after graduating from college and worked for the president's father, Fred, as an accountant. He has since become the Trump Organisation's chief financial officer and one of the president's closest business confidants (alongside Jason Greenblatt, who was Trump's in-house legal counsel before the president named him as a special diplomatic envoy to the Middle East).

"Over the years, Weisselberg's professional duties also came to include handling Trump's personal finances as well as the Trump Organisation's corporate finances. He has paid household bills, made large purchases for Trump, and has communicated with Trump's outside investment advisers. After Trump became president his lawyers created a trust that safeguards his interest in the Trump Organisation while ostensibly managing the company without his input. The trust is run by Weisselberg and the president's two eldest sons, Donald Jr. and Eric."

In other words, Mr Weisselberg has served as something of a jack-of-all-trades for Mr Trump. He worked for the Trump Organisation and the Donald J. Trump Foundation, yes, but he also handled personal stuff - up to and including tax returns . . . and apparently consulting with Mr Cohen on how to handle paying for the rights to a story about an alleged Mr Trump affair with a Playboy Playmate, just two months before the 2016 election.

Mr Weisselberg has now been tied to both the potential Ms McDougal payment (Mr Cohen never ultimately purchased the rights from the National Enquirer's parent company, American Media Inc.) and to the payment to adult-film actress Stormy Daniels, who has also claimed an affair with Mr Trump, in which he set up Mr Cohen's reimbursements from Mr Trump. That speaks to his deep involvement in All Things Trump and to the information he could provide investigators under the right circumstances.

Whether Mr Weisselberg actually knew the specific details of either arrangement isn't clear. We also don't even know if there was anything illegal about the whole thing - much less if Mr Weisselberg would have been involved in any illegality. So the idea that Mr Weisselberg might flip on Mr Trump is premature, at best.

But the fact that there was reason to subpoena him is big. That means he has to answer investigators' questions, save for ones where he might invoke the Fifth Amendment or attorney-client privilege. It also means the probe has clearly expanded into Mr Trump's business, which constituted Mr Trump's previously expressed red line.

"The subpoena is significant because it indicates that the probe encompasses the welter of transactions done under the umbrella of the Trump Organisation, with possible criminal liability for the organisation itself," said former Justice Department official Harry Litman. "Moreover, it means investigation into possible abuse of the organisational structure to serve Trump's personal purposes.

If he was truly the guy people like Mr Cohen sought to execute shady dealings like setting up companies for payments to cover up alleged affairs, just think of what he might know.