The bout proceeds despite Matias returning a positive test for Ostarine, a muscle-growth agent that is on the World Anti-Doping Agency banned substance list, in an out-of-competition sample taken on 9 November by the Voluntary Anti-Doping Association.
A B-sample also produced an adverse finding, but the New York State Athletic Commission cleared the 33-year-old to compete, ruling the concentration detected was below the threshold used by major boxing jurisdictions.
After the initial failed test, the WBC cited "mitigating factors" and concluded the low levels were consistent with accidental ingestion through contaminated supplements. Matias was, however, put on a 12-month probation period with the sanctioning body.
Although the failed tests were not addressed directly at the news conference, tension simmered when Matias addressed Smith's promoter, Eddie Hearn.
"If you were a fighter, your face would be smashed," Matias told Hearn.
"Thank God I'm not a fighter," Hearn replied. "But, as I said, justice shall be served."
When pressed by a reporter on what he meant, Hearn sidestepped the question and redirected attention back to Smith.