Saracens today ruled their World Cup lock Danny Grewcock out of England's Six Nations Championship opener against Ireland.

That could leave England coach Clive Woodward needing to find a new second row pairing, given World Cup captain Martin Johnson's worrying Achilles problem.

In-form Wasps forward Simon Shaw and Bristol's Garath Archer are the favourites to take over. Grewcock, who underwent groin surgery earlier this week, will be sidelined until at least mid-February.

He definitely misses the February 5 Irish appointment at Twickenham and must be extremely doubtful for England's trip to France a fortnight later, even though he hopes to return when Saracens tackle Premiership rivals Wasps in between those first two Six Nations internationals.

"Danny will be out for between four and six weeks, although knowing him that will be nearer four than six,'' said Saracens team manager Mike Scott. "He's had the injury for a while and initially we thought it would be possible to manage him successfully through the season.

"But during the Christmas period of matches it became clear that wouldn't be possible. It is a wear and tear problem."

Grewcock made a big impact on the Test scene last year, removing experienced British Lion Tim Rodber from England contention as he forged a powerful partnership with Johnson.

They were together for the World Cup matches against New Zealand and South Africa and were set to continue that association as Woodward's first-choice second-row pairing throughout this season's inaugural Six Nations campaign. Johnson last played on December 12 and is again missing from the Leicester line-up that faces Saracens at Welford Road tomorrow.

Publicly, Johnson remains optimistic about his chances of making the Ireland game. England, though, are aware that time is running out. Woodward is due to name a 26-man Six Nations squad next Thursday following a busy round of midweek Premiership action. Johnson and Saracens wing Dan Luger, England's World Cup top try scorer, are his biggest concerns.