Welsh rugby’s governing body has said it proposes to grant three licences for men’s professional clubs.
There will be one in Cardiff, one in the east and one in the west, which is expected to result in a straight survival fight between Swansea-based Ospreys and Scarlets in Llanelli.
Buchanan does not agree with the reduction in the number of sides after he pointed to the initial previous change in 2003 from clubs to regions.
“We came from our clubs to our regions and those were big steps,” said Buchanan.
“We had success on the field with Triple Crown, Grand Slams and Six Nations Championships in that era.
“The Scarlets, Cardiff, Ospreys, were all successful stories. So what brings me to this is what’s happened and how have we arrived here?”
The 70-year-old went on:
“What are we going to do to bring ourselves back into being a top tier one country?
“The next step made will be crucial. And I cannot support that next step in taking one of our top regions out.”
Buchanan believes losing Ospreys or Scarlets will be detrimental to the Welsh game.
“Whichever one goes, it’s going to be devastating,” he said.
“The Ospreys stretch from Bridgend, or Cardiff, down to Swansea and the Scarlets are down to the west with a new stadium and everything.
“I don’t see the decision being a step forward. It could be we could lose a lot of support on and off the field.”
The former Wales forward said he had also been against the WRU’s previous plan of two professional sides that was shelved following a public consultation.
“I was horrified with that thought,” he added.
“We are tribal in Wales. We like our local derbies and we like rugby in our areas. Those are important things to me.”
Buchanan also raised the prospect of the professional sides losing some control of the playing side of the organisation to the WRU.
“We have people that have been supportive of Welsh rugby, benefactors that have given their money,” said Buchanan.
“I can’t go into the detail of what is being offered to them, but it isn’t something they’re keen on signing.
“So we’re at loggerheads. It’s crucial we mend those fences and understand it’s a partnership. That’s what is missing at the moment, an agreement with a partnership.
“People have put their hands in their own pockets to put into the professional game and we need to respect that.”