It is perhaps the biggest mystery of the Premier League season: what on Earth has happened to Liverpool?

Manager Jurgen Klopp's so-called "mentality monsters" have become fragile and erratic, a team who can follow a 7-0 win over Manchester United with 0-1 defeat at Bournemouth, and a team who can lose 2-5 at home to Real Madrid yet fight back superbly to draw with Arsenal, as they did last week.

Klopp has looked dumbfounded with what he is watching as his "heavy metal" football has seemed like an out of tune quartet at times. Barring a sensational upturn in form, Liverpool will not be in the lucrative Champions League next season. They might not play in Europe at all.

Such an absence would severely hit the club's ability to attract the stars who might get this great club back chasing trophies and exciting fans again. Already, Liverpool have opted out of the race to sign England's dynamic £130m-talent, Jude Bellingham of Borussia Dortmund, preferring to use whatever funds are available on more modest targets such as Chelsea's Mason Mount or Brighton's World Cup winner Alexis Mac Allister, if those deals can be done.

- Stream on ESPN+: LaLiga, Bundesliga & more (U.S.)
- Read on ESPN+: Rating Europe's best impact subs

One thing is clear: this season has been a write-off by Anfield's recent standards and things have to change in the summer. But how?

Yes, there will likely be a clear-out with Alex Oxlade-Chamberlain, Naby Keita and James Milner all out of contract, while Arthur will return to Juventus. But these are all fringe players. Klopp must somehow pump renewed energy and intensity into his core group, nearly all of whom have been a long way short of the form that won them the Premier and Champions League titles in recent years.

However, Klopp has been trying for months to get to the root of the problem, apparently without success. He has looked dumbfounded and at a loss to explain some performances, promising only that he and his team will work hard at trying to rediscover the old stardust.

Even the previously imperious Virgil Van Dijk has come in for criticism, with Dutch legends Marco Van Basten and Ruud Gullitt saying he lacks leadership and "causes chaos." Harsh. Certainly, he has been left a little exposed with full-backs Trent Alexander-Arnold and Andy Robertson rampaging forward and Fabinho not looking like himself as midfield anchor, which allows opponents freedom to attack a porous defence.

One theory that might have some validity is that the players, after nearly eight years of Klopp's full-on style, may just be a little burned out. Last season's effort to win a "quadruple" has taken an emotional toll. (Liverpool fell short, winning the FA Cup and Carabao Cup, and falling barely short of the Premier League and UEFA Champions League titles.)

Certainly the stats seem to indicate a drop in intensity. Confidence has waned. Squabbles have broken out between players on the pitch. They are hurting to see a season that promised so much deliver so little.

Might those Liverpool players have also just stopped listening to Klopp? Not deliberately, of course. But it happens that over time, any manager-players relationship can become stale. They have heard it all before. That raises the question that dare not be asked, but possibly needs to be: for how much longer does Klopp himself get a free pass?

Of course, he has been one of Liverpool's greatest managers and nothing can ever change that. His teams have produced some electrifying football. But, as a deep thinker and clever man he will know that every boss has a certain shelf life at a club. You doubt there will ever be another 27-year reign like Sir Alex Ferguson at Manchester United.

The debate now must be whether, without the financial clout of some of his rivals, Klopp can produce another great team to emerge from the ashes of this nightmare season. You would not bet against it. Summer recruitment to freshen things up will be crucial, yet far from easy if there is no European football on offer. And with Liverpool languishing in eighth place a full 12 points off the top four, it will take more than just a retread of the current squad to stage a red revival on Merseyside. The current run of five games without a win underlines that point.

What happens next at Anfield will be fascinating because, no matter what, Liverpool can't keep carrying on like they have been this season.