The Phuket-based Scottish star will face Chinese newcomer Zhou Jiaqiang at ONE Fight Night 46 on Aug 15 at Lumpinee Stadium in Bangkok.
But rather than signalling a long-term pursuit of kickboxing gold, coach JP Gallacher said the bout had been arranged merely to keep ONE’s interim featherweight Muay Thai champion active.
“We have no plans to stay in kickboxing,” Gallacher told the Bangkok Post.
“This is just a one-time thing to stay busy. Thanks to the ONE team for finding us the opportunity.”
Gallacher said there were “two factors” behind the decision to cross over instead of defending Carrillo’s interim belt or waiting for a unification bout with Tawanchai, who remains sidelined by the broken leg he suffered against Liu Mengyang in December.
The first was the camp’s belief that Carrillo should already have been elevated to undisputed champion.
“We are not waiting for Tawanchai – we are waiting for ONE to do the right thing and strip Tawanchai,” Gallacher said.
“We and the whole fight scene are asking the same question: why has he not been stripped yet?
“For as far as I can remember, no champion has ever been allowed to hold on to a belt that long with no activity and no sign of a return.”
Carrillo claimed the interim title with a unanimous decision victory over Shadow Singha Mawynn in February, overcoming a difficult start before breaking the Thai contender’s jaw during their five-round battle.
The “King of the North” had already endured a lengthy wait for his opportunity after moving up from bantamweight and establishing himself as the leading challenger at featherweight.
“There are currently no Muay Thai contenders available for Nico to compete against, which ONE are currently working on,” Gallacher added.
“So fingers crossed ONE makes Nico undisputed [champion], and that will be the next thing the fans can look forward to.”
The 27-year-old Carrillo and his team had previously expressed frustration at Tawanchai’s pursuit of a second title in kickboxing while continuing to hold the featherweight Muay Thai belt.
The Thai superstar challenged Masaaki Noiri for the ONE interim featherweight kickboxing title in March 2025 but was stopped in the third round.
His last Muay Thai fight came two months earlier – a title defence against undisputed featherweight kickboxing champion Superbon, whom he finished in the second round.
Carrillo’s own move into kickboxing prompted speculation that he could eventually be added to the eight-man featherweight Grand Prix, which begins at ONE Samurai 2 in Tokyo on Aug 8.
The winner will receive a shot at Superbon, who is recovering after undergoing elbow surgery.
Gallacher, however, insisted Carrillo would not follow Tawanchai in attempting to become a two-sport champion.
The plan is to stay active against Zhou before returning to Muay Thai – either for a unification fight with a healthy Tawanchai or as the division’s elevated undisputed champion.
Tawanchai said after surgery that his recovery could take between nine months and a year. He may also require a tune-up fight before facing Carrillo, potentially delaying the unification bout further.
The 27-year-old Thai was present in the front row at Wednesday’s grand opening of the Evolve MMA gym at EmSphere in Bangkok, where he was photographed speaking with ONE chairman and CEO Chatri Sityodtong.
However, neither Tawanchai nor ONE has publicly announced a firm date for his return.