Ryan has damaged a hamstring tendon in his right leg and will require surgery.
It is the opposite hamstring to the one he injured, and had subsequent surgery on, in round three against Fremantle last season.
Ryan went down at training on Monday, continuing the rotten run last year's wooden spooners have had with injury.
"Unfortunately, Liam has suffered a high-grade hamstring injury with some tendon damage which will require surgery to repair," Eagles football boss Gavin Bell said.
"We are all really disappointed for Liam.
"He had been working hard this pre-season and over the Christmas break to be ready to go when we start playing practice matches.
"We're hopeful he will be available to return in the early rounds of the season.
"The club will provide an update on his return to play timeframe in due course following surgery."
West Coast coach Adam Simpson admitted Ryan's looming absence would hurt the Eagles in their rebuild.
"We need our best players available, not only to help win games, but to help grow our younger list as well," he said on Monday.
9°C