England and Northampton flanker Calum Clark is unlikely to play in the Six Nations, according to his club boss.
The 23-year-old suffered a shoulder injury against Glasgow and was set to undergo surgery on Wednesday.
"It is his ACJ (acromioclavicular joint) in his shoulder and he will be out for a number of weeks," Saints director of rugby Jim Mallinder told BBC Radio Northampton.
Calum Clark factfile
Born on 10 June
Joins Leeds academy at the age of 14
Becomes youngest forward to appear in the Premiership, for Leeds against Saracens
Wins a call-up to the England Under-20s; sent off in the final of the Junior World Championship against New Zealand
Signs for Northampton and captains the team, aged 21, in an LV Cup match against Saracens
Named Saints' young player of the season after making more appearances than any other Northampton player, including a start in the Heineken Cup final
Named in Stuart Lancaster's first senior England squad; banned for 32 weeks in March
"And I don't think he'll be back within the Six Nations timeframe."
Saracens lock Mouritz Botha was called up to replace Clark and joined up with England at their pre-tournament training camp in Leeds ahead of their Six Nations opener against Scotland on 2 February.
"It's a big blow whenever you lose players and I'm really disappointed for him with the Six Nations coming up," Mallinder added.
"It's well documented with him missing games previously, he came back from that strongly and he's been playing well for us."
England boss Stuart Lancaster said on Tuesday: "It is a shame for Calum as he had worked hard to get back into the senior squad.
"But these things happen and we'll work closely with Northampton on his rehab."
32-weeks suspended last season for breaking Leicester hooker Rob Hawkins's elbow
before the latest setback in his career.
"My shoulder needs an operation so I will be out of action for the foreseeable future. That's the game,"
Clark said on Twitter.