Garcia floored Haney three times in his upset victory, but did not win his American compatriot’s WBC light-welterweight title after missing weight for the contest.
Golden Boy Productions, Garcia’s promoters, said they will “determine how this finding came to be”.
“Ryan has put out multiple statements denying knowingly using any banned substances – and we believe him,” the statement added.
Haney told ESPN the result has “put the fight in a completely different light”.
“We learned about this situation not too long ago and it’s unfortunate Ryan cheated and disrespected both the fans and the sport of boxing by fighting dirty,” Haney said.
“Ryan owes the fans an apology, and by his recent tweet he still thinks this is a joke. We put our lives on the line to entertain people for a living.”
British boxer Amir Khan was handed a two-year ban by UK Anti-Doping after he tested positive for ostarine in 2022.