The Daily Mail says Boro chairman Steve Gibson blocked the Nigeria striker's attempt to secure a £10million move from Teesside and insisted he sees out the remaining three years of his contract.
With the clock ticking down to the start of the new Premier League campaign, Boro rejected that offer but were stunned that Yakubu, prompted by his advisers, demanded talks with manager Gareth Southgate the following day, claiming he was unhappy and wanted to leave.
That came less than a week after Southgate had a private meeting with Yakubu and sought assurances about the player's future. Yakubu said that he was content to stay.
A Boro source said: "It was almost laughable. Yakubu was happy one minute and then claiming he was unsettled and wanted to leave the next. He has been told in no uncertain terms that he is not for sale and he is not leaving.
"There is a risk he could sulk and be unsettled by all this but that's a risk they're going to have to take at this point of the summer."