West Coast will await scans to determine the severity of Jack Darling's ankle injury during the side's win over St Kilda.
The gun forward hurt himself during a marking contest in the first quarter, landing awkwardly on Saints defender Logan Austin's foot.
The 25-year old was helped from the ground by trainers and taken down to the rooms, and after riding a stationary bike, later emerged on crutches.
"I don't know how Jack is. He wasn't good enough to come back on, so it was another ankle injury, so we'll find out with some scans and hope for the best," coach Adam Simpson told reporters post-match.
"I can't rule anything out on Jack. I'm worried, but I always worry.
"I'm not a doctor. I'm not going to lie to you. I don't know how he is.
"I haven't spoken about how he's feeling.
"We'll find out tomorrow or the next day."
Simpson said the club's medical staff tried to get Darling back onto the field, but had little success.
"I think they always try to do that. With ankles it's 'can you walk on it? Can we help it? What do we think it is?'" he said.
"When they don't know what it is they go 'let's just shut it down'. So that's what happened."
Darling has been in career-best form in 2018, booting 28 goals for the top-of-the-table Eagles, which ranks third in the AFL.
He has also averaged a career-high 7.8 marks, including 3.5 inside 50. His 31 contested grabs this season rank first in the AFL, putting him firmly in the frame for All-Australian.
West Coast have a bye next week, followed by clashes with Sydney (SCG), Essendon (Optus Stadium) and Adelaide (Adelaide Oval).