Earlier this year, the WRU had previously stated there was enough money to fund four professional sides equally.
Tierney says things changed when Cardiff went into administration in April and was taken over by the WRU.
The current professional rugby agreement (PRA) that underpins the Welsh professional game runs out in June 2027 and was due to be superseded by a new five-year deal.
The deal has been agreed by Cardiff and Dragons, but has still not been signed by either Ospreys or Scarlets, who say “key issues” have not been resolved.
“When Cardiff happened, that showed just how challenged the system is,” said Tierney.
“It made us all pause, including the regions. That meant we weren’t able to give sufficient reassurances to two of the regions for them to sign.
“So when you don’t have a deal, it’s time to think about what to do different.”
Tierney say a drop in finances has affected their previous forecasts.
“When we modelled the original offer, it was based on the numbers at the time but numbers have gone the wrong way since then,” said Tierney.
“We should have done a tougher downside but the headwinds that have affected rugby globally since were not reflected, especially competition income and media rights, while costs have also gone up with national insurance, player wages and costs of running a stadium.
“We’ve seen revenue going down and costs going up. It means the gap to make up is larger for the regions. There’s only so far the money goes.”
Ospreys and Scarlets are considering legal action against the WRU about their handling of the Cardiff situation.
“That’s always a concern,” said Tierney.
“They are all still talking to me about what’s in best interest for Welsh rugby and want to find a way forward.
“I remain optimistic we will find a way through this and can avoid legal action because that’s the worst thing that can happen for Welsh rugby with delays and costs involved.”
