Thompson said the university requested a medical exception from the NCAA so Trier could continue playing, but that it was rejected Thursday before Trier was ruled ineligible.
Trier sat out the first 19 games in 2016-17 after testing positive for the drug, which he said came from a well-intentioned person not affiliated with the university following an injury.
Trier has been among the nation's best guards this season, averaging 19.6 points, 3.2 rebounds and 3.2 assists per game with remarkable efficiency. He has started 27 games this season and is indispensable enough to the Wildcats - who are ranked No. 14 and leading the race for the Pac-12 Conference championship - that he plays 34 minutes a game at both point guard and shooting guard.
Arizona will be without him as they face the Oregon State Beavers on Thursday night.
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Arizona is appealing the suspension for the positive test, which the school argues is not a result of renewed PED use, but a residual trace of the steroid Ostarine in Trier's system from 2016.
Arizona will appeal the decision but for now it appears that UA's chances of making it to the Final Four in April have taken a giant hit.
According to ESPN, Trier has said that he never "knowingly" took the banned substance. Trier tested positive for a performance-enhancing drug last season, which resulted in a lengthy suspension. "The NCAA agreed that I had no knowledge of receiving the substance and my eligibility was restored". It's unclear whether the NCAA's same standards would apply following Trier's latest missed test. "This is so frustrating and unnecessary, especially at this time of the season and Allonzo's career".
A college basketball star has been ruled ineligible.