Davis sprained the same shoulder in the Crows' loss to Fremantle in round three and lasted only a quarter into his comeback game against the Saints before re-injuring the joint.
The former NAB Rising Star nominee passed a rigorous fitness test at training earlier in the week, but his shoulder popped out with minimal contact early in the second term when he tackled Saints skipper Nick Riewoldt.
He was helped from the ground clutching his arm and was immediately substituted from the match.
Davis had a reconstruction after injuring his shoulder in an Under-18 game before to being drafted by the Crows in 2008.
The former first-round draft-pick is the third Crow alongside Jason Porplyzia and David Mackay to suffer a shoulder injury this season.
He will have scans next week, but coach Neil Craig said the prognosis wasn't good.
"It's been dislocated, so it's not good news," Craig said after the match.
"What that means long-term, whether it's rehab or surgery, we won't know until early in the week.
"It's disappointing for Phil because he's a key player for us and he's developing into an important player. I love his attitude to footy and it's just disappointing for our supporters when that sort of injury occurs."
The injury to Davis forced Craig to switch inexperienced defender Luke Thompson onto Riewoldt.
Thompson showed great poise for a player in only his third AFL game and Craig said he had been pleased with the former rookie's performance.
"Luke was good. He went to the back pocket tonight, but had to come out and take over the Riewoldt role," he said.
"It was a great experience for him."
The Crows dominated the first half and led by 30 points early in the third quarter, but appeared in danger of relinquishing a potentially match-winning lead for a third straight week when St Kilda closed to within three points at the start of the final term.
However, the home side improved on its panicky performance in the dying stages against Carlton last weekend and kicked three unanswered goals to secure the important win.
"Did we learn from last week? You'd hope so. You’d hope you would learn something from that, just to keep a bit calmer about where we were going," Craig said.
"We certainly looked calmer this week in the position we were compared to last week. We looked a bit stargazed towards the end of the game against Carlton.
"Coaches talk about learning from experience. How much we learned I'm not sure, but maybe it was good to go through that experience against Carlton - a really powerful side as it's turning out - and maybe the guys took a bit a little bit of that into the end of the game tonight."
