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.

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.