Thilthorpe landed awkwardly in a marking contest at Richmond Oval on Saturday, then sat on the ground momentarily before hobbling off, assisted by a medico.
He was sent for scans on Sunday, which showed damage to the meniscus in his left knee.
Thilthorpe will consult with specialists in the coming days before the club can put a timeline on his return.
"Sometimes your heart is in your mouth. Today was one of those moments," Crows coach Matthew Nicks said post-match.
"But we have got a squad of players too. Riley is really important to what we're doing and he's a great player, but we've got others.
"And that's part of pre-season. We train on the rivet, we go hard.
"That's how we've trained right throughout, so we've been pushing the envelope, let alone playing games like this.
"And we'll continue to do that. That's how we work, that's how we believe we play our best footy."
Adelaide don't feature in next weekend's opening matches of the season, playing their round one match at the Gold Coast the following Saturday.
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