He had been expected to return to action at the start of next month, but the latest assessment on his injured knee suggests he will be forced to miss out for another few weeks.
"Mike had a scan this week, which has confirmed that his knee has reacted to the high level of rehab he has gone through over the last few weeks," said Ospreys physiotherapist Chris Towers.
"As a result of this, we have to accept that he needs to rest up over the next week or two to allow the knee to settle down.
"The specialist is optimistic that Mike will be able to pick up where he left off once the knee has settled down but, looking at the situation realistically, he will still need a few weeks to work on his rugby fitness before we can consider him ready for competitive rugby.
"It's very disappointing that this hiccup in the final stages of his recovery has ruled him out of contention for the opening rounds of European rugby, but it is in the long-term best interests of Mike, the Ospreys and Wales that we ensure his knee is 100 per cent right before he returns to action."
The delay in Phillips' return means he is highly unlikely to have played any competitive rugby prior to the October 21 announcement of Warren Gatland's squad for the autumn internationals.