In the immediate aftermath of Scottish Premier League clubs rejecting Charles Green's application for top-flight entry, Regan also made a stark claim that depriving Rangers fans of their team would spark "social unrest".
Regan said: "The only solution for the game now is that Rangers come into the Scottish Football League and they come into it in the First Division. If Rangers were to go anywhere other than the First Division, then there would something in the region of £15.7 million worth of losses to the game."
Regan made it clear the SFA could not allow the Ibrox club to start at the bottom, meaning Scottish Football League clubs face the prospect of an SPL2 breakaway if they reject a plan to put them in the First Division.
He added: "For the bigger clubs at the top of the league, that's half their annual distributions. For clubs at the bottom it is basically wiping out their entire distributions, for some of the smaller clubs it's a huge proportion of their annual turnover.
"Even if Rangers end up in the First Division, there is still going to be a £5m loss of income to the SPL clubs. The game is not sustainable so there would be a slow lingering death for the game in Scotland.
"It would then trickle down to the SFL. From our perspective as the governing body, we cannot allow that to happen."
The SFA chief executive also confirmed a real threat to television contracts.
"We have had dialogue with the broadcasters and we understand what the various stakeholders from Sky television, ESPN, Sport Five and a number of the SPL's other commercial partners are likely to do in the event Rangers are not in either of the top two tiers," he said. "It's not pretty.
"That's why we cannot sit back and let that happen without trying to get all parties to accept this is the only solution which can keep the game afloat. Without Old Firm games, the value drops, the overseas deals are almost exclusively about the Old Firm derby and that would go immediately."
Meanwhile the Gers have been left without any pre-season matches after organisers of their tour of Germany cancelled the trip.
A statement on the club's website read: "The Rangers Football Club has been informed by the organisers of our pre-season trip to Germany that they can no longer offer us a place on the tour.
"The team was due to play three games in Germany between 18-25 July, but the trip will now not take place at their request."