Rioli's season is over after he was slapped with a provisional ASADA ban on Wednesday.

He could face four years out of the game if an investigation finds he substituted another substance for his own urine during an August 20 out-of-competition drug test.

The 24-year-old posted a message on his Facebook page on Friday apologising for the controversy, while also vowing to learn from the distressing experience.

"Just want to thank all my real family and friends for all the love and support through this tough time," Rioli wrote.

"I'm so so sorry I had to put (you) all through this. We will get through this. I've gone through tougher times life support, depression, bullying and we've gotten through.

"Hopefully this lil mistake is a lesson to be learnt. This definitely doesn't define me as a person.

"Love (you) all."

A devastated Rioli left the team hotel in Melbourne after he was informed of the breach of the ASADA code and flew to Darwin.

He will be represented by David Grace QC in what AFL chief Gillon McLachlan expects to be a lengthy investigation.

It has been reported an energy drink could have been used in place of urine from Rioli, but Eagles football chief Craig Vozzo would only say "something other than urine" had been detected in his sample jar.

Rioli was removed from the line-up to face Geelong in Friday night's semi-final at the MCG following the revelation.

McLachlan and West Coast have expressed their concerns for Rioli's mental health, but the AFL boss said that the joint AFL-ASADA investigation must be given time to run its course.

"Technically there are provisions for an expedited hearing but my understanding is (it won't be)," McLachlan replied on 3AW on Friday when asked if there was any chance the investigation could finish before the grand final.

"The priority for West Coast and for us is Willie's mental health ... that's not something that's in contemplation.

"He has been provisionally suspended while ASADA continues their investigation and the case comes to a head.

"These things take some time and this is clearly something that's different from the standard ones so I'm sure it will take some time."

Collingwood's Sam Murray was recently handed an 18-month doping ban arising from a test that occurred in August last year.

Rioli can train with the team during his provisional suspension, but can't play in the WAFL or any other league that is signed up to the World Anti-Doping Agency code.

Questions have been raised over the testing process and how a drink could possibly be used as a replacement without ASADA testers present raising the alarm.

"I'm cautious ... but I think they (ASADA) were only clear when the final results came in on Wednesday that it wasn't urine," McLachlan said.

McLachlan would not say whether Rioli has been tested again after the alleged tampering took place or whether he has any strikes to his name under the league's illicit substance code, which is separate to the ASADA code.

Australian Associated Press