"It is clear that perception and reality have become horribly mangled," said Coe after an IAAF Council meeting yesterday.
"I've stepped down from my ambassadorial role with Nike, which I've had for 38 years. The situation was "not good for IAAF and not good for Nike".
Coe, unpaid as head of track and field's world governing body, received around 100,000 pounds (142,000 euros) a year for his global ambassadorial role for Oregon-based Nike.
Bjorn Eriksson, who led a rival bid by Gothenburg for the 2021 championships, said Coe telephoned him on Wednesday to say it had been wrong to give the event to Eugene without a formal bidding process, The Times reported.
More From This Section
Coe insisted, however, that he had not been responsible for the decision that was made in April.
"I don't believe it was a conflict of interest," Coe stressed.
The decision to step down from Nike "was purely on the need to focus on challenges ahead with my colleagues and particularly the executive teams here at (IAAF) headquarters", he said.
Coe added, however, that he had sought advice from the IAAF Ethics Commission, who said the Briton could have continued in his Nike role as long as he "clearly and consistently declared all my interests".
Eriksson also said Coe had indicated that the Eugene award was being investigated by French police as part of a corruption inquiry into the IAAF leadership of Diack, who stood down in August.
Diack is also under investigation over allegations that he took bribes from Russian officials to cover up positive drug tests by athletes.
Coe had been a strong supporter of Eugene's bid for the 2021 championships and was part of the IAAF council that voted this year to abandon the normal bidding process.
Nike, which was founded in Eugene, was also a powerful backer of the bid.
And the BBC said Tuesday it had seen an email in which a Nike executive said Coe had assured him he would "reach out" to Diack on behalf of Eugene.