The former 100 metres record holder tested positive for a banned stimulant at June's Jamaican Championships while fellow Jamaican Sherone Simpson also failed a drug test at the same event.
Canadian Xuereb has rejected claims from Powell's agent Paul Doyle that the results were due to supplements he had provided.
"I did not provide any banned or illegal substances," he said in a statement.
"I am extremely disappointed that these athletes have chosen to blame me for their own violations."
Paul Doyle had indicated to the New York Times earlier in the week that Xuereb was behind the supplements that led to the athletes testing positive.
"We were trying to figure out what went wrong and it was pretty obvious to us where we needed to look," he said.
"There are many different things he was giving them and we still don't know which one caused the positive test. Most of the supplements he gave were for recovery or energy during workouts."
Powell is awaiting the results of his B test along with former world champion Tyson Gay, who last week was told that his A sample from an out-of-competition test in May was positive.