England captain Dylan Hartley will return to action on Saturday after a six-week absence - but head coach Eddie Jones has been dealt another injury blow.
Hartley will line up for Saints in their European Champions Cup clash against Castres, having been sidelined since mid-September due to back trouble.
But Saracens revealed on Friday that lock George Kruis has undergone "minor ankle surgery" and is expected to be sidelined for "a number of weeks."
England boss Jones, whose team have reeled off nine successive victories since he took charge, has been hit by a lengthy injury list.
Anthony Watson (fractured jaw), James Haskell (foot), Manu Tuilagi (groin), Jack Clifford (ankle), Mike Williams (broken arm) and Sam Jones (broken leg) have already been ruled out of England's four-Test autumn series, which starts against South Africa at Twickenham on November 12.
Exeter pair Jack Nowell and Luke Cowan-Dickie are also currently on the sidelines and it now looks as though Kruis, who has been among England's stand-out performers under Jones, could miss the autumn games.
Saracens' rugby director Mark McCall said: " After consultation with George and England, we are all in agreement that it's in the best interest of the player to undergo this minor procedure now with the majority of the season still ahead of us.
"George is on the road to recovery and we look forward to his return to action sooner rather than later."
The consolation for Jones is that England have outstanding second-row strength in depth, with Wasps' Joe Launchbury and Northampton's Courtney Lawes among alternatives to Kruis as a prospective partner for current European player of the year Maro Itoje.
Itoje, though, could also be considered for a back-row spot, given his versatility and England's current injury problems at openside flanker.
Hartley, meanwhile, goes straight into Northampton's starting XV this weekend, having last played when Saints beat Aviva Premiership opponents Bristol at Ashton Gate on September 11.
Source: PA
