Scrum-half Davies underwent a scan on Tuesday morning after picking up a knee injury early on against Newport Gwent Dragons on Sunday.
The 25-year-old is likely to be out for the next couple of weeks at least - ruling him out of the upcoming Thomond Park trip and the European opener in Toulon on October 19.
Back-rower McCusker will also miss the Munster match because of a shoulder injury, and the length of his absence will also be determined by a scan.
"Gareth has had a scan but he will be out for the next couple of weeks at least," Pivac told Wales Online.
"As for Rob, he has some damage to his shoulder, so we'll just have to see how bad that is."
There is some positive news for Pivac, though. Kiwi centre Regan King, who hasn't played since the opening match of the season against Ulster because of a hamstring injury, is on the road to recovery.
"We have re-scanned his leg and it showed that it has healed up," added Pivac.
"Regan has been running and we hope he can be back for the early rounds of Europe, probably for the Leicester Tigers game."
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