James has returned to the UK and will visit two specialists to fully assess the injury which is to the same (right) shoulder which kept him out of action for 10 weeks at the beginning of 2008.
"Now that Butch has retuned he will see a specialist in Bath tomorrow and one in London on Monday," said Bath coach Steve Meehan.
"The guy in London is the one who has dealt with him in the past and that will be the best indicator. Once he gets there we will have a much better idea of where he sits."
The former European champions face Biarritz Olympique in their first pool game before travelling to Aironi Rugby in Round 2.
And while the news is clearly disappointing for Meehan's men, the form of fellow fly-half Sam Vesty will help lighten the blow after the ex-Leicester star scored 17 points in his competitive debut at the weekend.
"I suppose you always fear when someone goes down on the paddock," added Meehan.
"But we have a strong squad at inside backs and we are confident that we would be able to handle his absence for whatever period of time."
After facing French and Italian opposition in October, Bath will be looking for revenge over Pool 4 opponents Ulster when the two sides meet at Ravenhill on Saturday, December 11.
Ulster, who won the Heineken Cup in 1999, a year after Bath achieved the same feat, completed the double over the West Country outfit at the same stage last season.