Injury-plagued midfielder Elliot Yeo is, meanwhile, set to miss the next three to four weeks after suffering a hip complaint during the club's 171-point loss to Sydney.
Waterman spent more than a week in hospital earlier this month and it has since been revealed the 25-year-old is battling ulcerative colitis - an inflammatory bowel disease that is an autoimmune condition.
The 84-game forward is almost certain to miss the rest of the season and there's a small chance the disease could end his AFL career.
Waterman's dad - two-time West Coast premiership player Chris Waterman - says doctors are hopeful his son will be able to play again.
But he criticised West Coast's lack of communication after his son flew home on the morning of the club's June 10 clash with the Crows in Adelaide and checked into hospital two days later.
Chris Waterman said the club took too long to check in on Jake's wellbeing - something the Eagles vehemently deny.
Simpson said he tried to make contact with the player while he was in hospital, but to no avail.
"A few text messages here and there but no response," Simpson said.
"He was quite ill at the time.
"I was unaware (of how serious it was) ... in the early stages and then it got quite severe and I still don't know the full extent of the situation.
"But it's not great. Our heart goes out to him."
When asked if he has chatted to the 25-year-old since then, Simpson replied: "Not properly, no. It's been a challenge."
Waterman is out of contract at the end of the season and could seek a fresh start elsewhere given West Coast are in the early stages of what looms as a long and painful rebuild.
Simpson has also been hit with another big setback after Yeo was ruled out of action for up to a month after scans showed up irritation around his hip flexor on his left side.
In another blow, star forward Liam Ryan is unlikely to play again this year.
Ryan tore his hamstring while attempting a high mark in the round-three western derby loss to Fremantle.
"It's looking more unlikely with the severity of that hamstring," Simpson said.
"It was always going to take a long period of time."
Former skipper Shannon Hurn (hamstring) and forward Jamie Cripps (ankle) are on track to return for Sunday's clash with St Kilda at Optus Stadium.
Defender Tom Barrass has been cleared to play despite copping a knock to the head in last week's 171-point loss to Sydney.
Simpson said Barrass had now passed three concussion tests.
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