West Coast's Willie Rioli could face a ban of up to four years after the AFL announced the Eagles premiership star had returned an adverse analytical finding for urine substitution following a doping test last month.

Key points: The AFL was informed by ASADA that Willie Rioli had "returned an adverse analytical finding" for urine substitution

The AFL was informed by ASADA that Willie Rioli had "returned an adverse analytical finding" for urine substitution Rioli was withdrawn from the Eagles' squad to face Geelong in Friday night's semi-final as a result of the provisional suspension

Rioli was withdrawn from the Eagles' squad to face Geelong in Friday night's semi-final as a result of the provisional suspension West Coast said it would continue to support Rioli, despite its disappointment in the premiership forward

The AFL has provisionally suspended Rioli, which means he has been withdrawn from the Eagles squad for Friday night's semi-final against Geelong at the MCG.

The potential violation of the AFL's anti-doping code was detected after an out-of-competition doping control test on August 20, with ASADA informing the league on Wednesday.

Rioli's A sample did not return evidence of a banned substance in his system, however the finding was that there was evidence of urine substitution, which is a prohibited method.

The provisional suspension will last until the matter is determined, with the doping offence carrying a maximum suspension of four years.

"A sample returned an adverse analytical finding for urine substitution, which is a prohibited method under the code," AFL general counsel Andrew Dillon told a media conference in Melbourne.

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

"Under the Code, ASADA conducts testing for the AFL as part of a 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-compliant sport [including the WAFL] while provisionally suspended, but will be able to train, prior to the hearing and determination of the asserted Code violation.

"Rioli has been withdrawn from the West Coast Eagles team that will play against Geelong tomorrow night."

Eagles left 'extremely disappointed'

Rioli's provisional suspension has the potential to rock the Eagles' premiership defence.

West Coast general manager of football Craig Vozzo said Rioli was a "bit of a mess" following the revelation about the test result.

"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 the staff are really worried about his well-being. He's not in a great space and it's our job to help him as much as we can."

Rioli is alleged to have used a prohibited method to provide a urine sample. ( AAP: Richard Wainwright )

The club had earlier released a statement, saying it would support Rioli despite its disappointment in the 24-year-old.

"The Eagles have been made aware that Willie Rioli is the subject of an ASADA and AFL investigation," the statement read.

"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."

The AFL Players' Association also offered its support to Rioli following the handing down of the provisional suspension.

"It is inappropriate to speculate on details of the case, or pre-empt the outcome," the statement read.

"The AFL Players' Association will continue to provide support to Willie and the club throughout the process."

Rioli has played 38 senior matches for the Eagles, having joined the club as the 52nd overall selection in the 2016 draft.