The 54-year-old, who had surgery for throat cancer in March, added he had not left the Thailand job to join a new club before next season after resigning from his post on Wednesday.
"I've got no plans," he told reporters when asked if he would take another management position.
"The only thing I have planned is to go back and I'm speaking to Man United about going full-time as an ambassador."
"It's not like I'm leaving to go into another management job. I have my reasons to leave and it's not to manage another team." he said.
Robson departed by mutual consent after 21 months in the post, ending a disappointing international coaching debut that saw Thailand slide 15 places in FIFA's rankings to 120 and fail to reach the Asian Cup finals for the first time in 23 years.
The ex-Middlesbrough, West Bromwich Albion, Sheffield United and Bradford City manager is hoping for a role at Manchester United where he made 345 appearances in a 13-year career.
SURPRISE RESIGNATION
Robson's resignation as Thailand coach was a surprise after he expressed a desire to continue working with the national team despite their miserable run of form last year.
But the timing has sparked speculation in Thailand that it could be linked to a row between Football Association of Thailand president Worawi Makudi and the English FA, which was instrumental in Robson moving to Thailand in September 2009.
FIFA Executive Committee member Worawi had enjoyed warm ties with England's football bosses but last month was accused by former FA chairman David Triesman of promising to vote for England's 2018 World Cup bid in return for television rights to a proposed friendly between the two national sides in Bangkok.
Worawi, who had threatened to sue Triesman, was cleared of any wrongdoing in an independent report commissioned by the English FA .
Robson, in a statement issued later on Thursday via Thailand's football association, said he had decided to quit as manager "primarily for health and family reasons..
"I leave with nothing but respect for the endeavours of people within the game who are working so hard to develop the game to reach the standard that it truly deserves," he said.
Worawi said no decision had been made on whether Robson's replacement would be a foreign or local coach.
Thailand start their 2014 World Cup qualifying campaign against Afghanistan or Palestine on July 23.
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