Will Greenwood will be out of action for at least four weeks after undergoing an exploratory operation on his shoulder - ruling him out of the remainder of the Pool Stages.
The news is a bitter blow to Harlequins fans, who are still searching for their first win in this season's Heineken Cup.
"Will underwent a scope on a long-standing shoulder problem," said Harlequins' Mike Scott.
"We are still waiting to hear from the consultant but he will be out for a minimum of four weeks, maybe longer."
Four weeks out would leave the Quins star only one club game before England start the Six Nations against Wales.
Greenwood, who helped his side raise the Parker Pen Challenge Cup, has struggled this season, but is still a devastating attacking force when on song.
