Ospreys and Wales prop Aaron Jarvis faces six weeks out after tearing a pectoral muscle to compound the region's injury worries.
Jarvis suffered the injury during a substitute appearance for Wales in the 12-6 win over South Africa.
Number eight Dan Baker is "unlikely to feature" until the new year because of a shoulder injury.
Fellow back-rower Joe Bearman and lock Lloyd Peers are sidelined with abductor muscle and ankle injuries respectively.
Jarvis' injury is a particularly heavy blow for Ospreys, who are already without Wales international props Nicky Smith, Ryan Bevington and Duncan Jones.
"The scan has confirmed that he doesn't require surgery, but he is still looking at a recovery period of around six weeks," said Ospreys medical performance manager Chris Towers.
Baker suffered nerve damage to his shoulder during the 18-12 defeat to Leinster in November.
Towers added: "In his situation it is a case of ongoing assessment and we are unable to set a return to play date at this point, but he is unlikely to feature again before the turn of the year."
Bearman's injury is expected to keep the former Newport Gwent Dragons player out for six to eight weeks, though the extent of Peers' injury is not yet known.
"He has had a scan which confirms damage to ankle ligaments," said Towers.
"We will now discuss with an ankle specialist the options for his treatment over the next few days, before deciding what is the best one for Lloyd."