Bath have ruled out surgery for the 24-year-old, who suffered medial ligament damage against Northampton.
Wales play Australia on 5 November before games against Argentina, Japan and South Africa.
"He's had a scan. It's anything from six to eight weeks," said Bath director of rugby Todd Blackadder.
Faletau, who joined the Premiership club from Newport Gwent Dragons this summer, was replaced 12 minutes into the 18-14 win at Northampton Saints.
He is one of seven Welsh exiles who will have to rely on being picked as wildcards for international selection during 2016-17.
George North, Jamie Roberts, Rhys Priestland, Dominic Day, Nicky Thomas and Rhodri Williams are the others.
The Welsh Rugby Union's senior player selection policy means only three players who play outside Wales can be picked.
Interim Wales head coach Rob Howley announces his squad for Wales' November Test matches on Tuesday, 18 October.
Wales number eight Taulupe Faletau is a doubt for the autumn internationals as he faces up to eight weeks on the sidelines because of a knee injury.