Theories abound about what has caused Liverpool's slump into mediocrity.
The ageing of Anfield stalwarts Steven Gerrard and Jamie Carragher.
The implosion of Rafa Benitez.
The catastrophic stewardship of Tom Hicks and George Gillett.
The global economic crisis that brought the club's future planning around a new state-of-the-art stadium to a standstill.
All these, and many more, are factors in Liverpool Football Club's recent struggles.
But there is also one very specific event that can be identified as the catalyst for many of Liverpool's problems on the pitch—the departure of Xabi Alonso.
During Alonso's time at Anfield, Liverpool won the Champions League and the FA Cup and strung together solid top-four Premier League finishes consistently.
After the Spaniard left for Real Madrid in 2009, Liverpool finished seventh. The top four has eluded them ever since.
Alonso's passing technique, vision, intelligence and impeccable distribution were sometimes underappreciated. But suddenly lacking a player with those attributes exposed Liverpool's limitations dramatically.
While the emergence of Lucas Leiva helped paper over some of the cracks left by Alonso's absence, the Brazilian is not a perfect replacement.
Can Nuri Sahin replace Xabi Alonso for Liverpool?
Loanee Nuri Sahin just might be.
The Turkish midfielder, ironically, owes his spell at Anfield to Xabi Alonso in more than one way. Sahin has been unable to break into the Real Madrid midfield so expertly marshalled by Alonso. And Alonso, always fondly reminiscent of his Liverpool days, recommended the club to Sahin ahead of Arsenal.
Sahin has the skills to emulate Alonso's role at Anfield. He should fit in seamlessly with Brendan Rodgers' passing philosophy.
And just easing into the side, he has already shown he can chip in with important goals, just as Alonso did.
But perhaps most important, Sahin can breathe new life into Steven Gerrard's career.
Gerrard's effectiveness has been decidedly mixed since Alonso's departure. There have been spells when Liverpool enjoyed better results without their talismanic captain than with him in the side.
It is possible this dulling of Gerrard's edge has been due simply to age and loss of form, or to inconsistent tactical approaches as Liverpool has struggled to settle into a coaching vision.
But it seems likely that Gerrard will benefit from Sahin's ability to orchestrate and release forward runs, as he did to devastating effect with Shinji Kagawa at Borussia Dortmund.
Nuri Sahin seems an almost ideal candidate finally to replace Xabi Alonso.
But as Brendan Rodgers pointed out, it could be dangerous to rely on Sahin as a long-term replacement—there is no guarantee Real Madrid will be willing to sell him on completion of his loan.
Or, crucially, that Liverpool could afford him.
Unfortunately, the better Sahin performs on loan, the less likely a permanent deal could be done.
Intriguingly, there are signs José Mourinho could be ready to displace Xabi Alonso from the Real Madrid midfield.
Alonso, a World Cup and double European Championship winner still near the peak of his career, would hardly welcome extended stays on the bench, and he has always been open to a Liverpool return.
Might Xabi Alonso ultimately return to Anfield to replace Nuri Sahin?