Liverpool will have to continue their pursuit of Champions League qualification and the FA Cup without Daniel Agger after the influential defender fractured a rib in the Carling Cup final defeat of Cardiff City.

Agger has withdrawn from the Denmark squad to face Russia in a friendly on Wednesday and could be out for the next four weeks with the injury that forced his substitution in the 86th minute of Sunday's win at Wembley. Liverpool have not placed a timescale on the 27-year-old's recovery as they wait to discover the full extent of the problem. The club are also awaiting an assessment on Glen Johnson's fitness after he withdrew from the England squad and he may miss Saturday's visit of Arsenal to Anfield.

The Denmark international suffered a similar injury earlier in the season at Tottenham Hotspur and it was four weeks before he returned to the Liverpool squad, five before he played for Dalglish's team again. Another four-week recovery period would rule the defender out of Premier League games against Arsenal, Sunderland and Everton – with the Merseyside derby at Anfield having been rearranged for Tuesday, 13 March following its postponement due to the Carling Cup final – plus the FA Cup quarter-final with Stoke City on 18 March.

Jamie Carragher replaced Agger at Wembley and, though Liverpool have their experienced vice-captain available for a demanding spell, it is a setback to Dalglish that his first-choice central defensive pairing has been disrupted. Agger and Martin Skrtel have formed an outstanding partnership this season in a defence that has conceded just 23 goals in 25 Premier League games – only league leaders Manchester City have a better defensive record.

The Liverpool director of football, Damien Comolli, has claimed the Carling Cup victory will help lure "big players" to Anfield this summer, with Europa League qualification assured by Sunday's result but secondary to the ambition of returning to the Champions League next season.

"We already know there are big players who want to join us because they believe in the project and what Kenny and the club are trying to do," said Comolli. "The fact we are playing in Europe and have won a trophy are all positive things. There is a positive vibe around the club around Europe."

Comolli also believes that Liverpool have the structure in place to fulfil Dalglish's post-final wish for sustained success. He added: "The final was a bit of a lottery because you can score a penalty or miss a penalty and you are a hero or a zero, which is unfair. It is overall you judge the performance of the club and the progress – not just the penalty which goes in or out.

"What you want is a progression and I think for the last 12-14 months we are showing progression everywhere on and off the pitch in terms of squad structure, the players Kenny has to choose from, the young players the academy is producing and who are coming through."