WEST Coast's Willie Rioli has been provisionally suspended in relation to an ASADA test in late August and faces a ban of up to four years.

The AFL late Thursday stood down the premiership-winning forward from Friday night's semi-final against Geelong at the MCG. 

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Rioli was advised of an adverse finding in the ASADA test - taken on August 20, two days after the Eagles' round 22 match against Richmond.

Eagles football boss Craig Vozzo shed further light on the alleged offence when he fronted a hastily-convened press conference in Melbourne on Thursday evening, saying the 24-year-old was 'a bit of a mess' following the revelation.

"It's something other than urine that has been produced by Willie as part of the testing process," Vozzo said.

"He's very, very flat.

"The club and our staff are really worried about his wellbeing.

"He's not in a great space and it's our job to help him as much as we can."

ASADA has alleged Rioli used a prohibited method to provide a sample for the test.

"The assertion is that the adverse analytical finding is that there's urine substitution," AFL general counsel Andrew Dillon said.

"ASADA informed the AFL Wednesday of the results of the testing of the sample and Rioli was formally notified late last night, when he came to Melbourne," Dillon said.

"Under the code, ASADA conducts testing for the AFL, as part of the comprehensive anti-doping program designed to protect the integrity of the AFL competition.

"The provisional suspension is effective immediately. Rioli will not be eligible to compete in AFL competitions or any WADA-compliance sport, including the WAFL, while provisionally suspended. He will be able to train prior to final determination of the asserted code violation."

Earlier, West Coast said in a statement: "The Eagles have been made aware that Willie Rioli is the subject of an ASADA and AFL investigation. 

"The club is extremely disappointed that one of our players has been provisionally suspended while a matter is being investigated and it will comply fully and openly with ASADA and the AFL through the process.

"The club is fully committed to supporting Willie during what is a personally distressing time.

"Our primary concern is Willie's health and welfare.

"As this is an ongoing investigation the club, its staff and players can make no further comment until the investigation is completed.

"The West Coast Eagles fully supports the AFL and ASADA in its ideal of our game being drug free."

Rioli's shock suspension is a significant setback for the Eagles on the eve of their do-or-die final against the minor premiers.

"Our players are very emotional, they love Willie," Vozzo said.

"But we've got a pretty mature team and I'm confident they'll be able to package their feelings for a couple of hours and perform at their best."

Rioli has played 38 AFL games, including the 2018 grand final win over Collingwood, since he was the 52nd overall selection at the 2016 draft.