Jurgen Klopp and Liverpool face challenging summer with work to do in the transfer window

Whisper it quietly, but this could very well be the most important summer of Jurgen Klopp's Liverpool tenure.

After flying so close to the sun in pursuit of quadruple immortality, Liverpool's altitude dropped significantly in the final week of the campaign as they finished second in the Premier League to Manchester City before losing the Champions League final to Real Madrid in Paris.

With an FA Cup and League Cup added to their overall haul at Anfield - which caused two alterations to the famous champions' wall outside their famous old stadium - there are few who are brave or foolish enough to declare Liverpool's season as a failure.

After all, with just four defeats from 63 games - one of which was an inconsequential second-leg loss to Inter in the Champions League - you have to be an extremely one-eyed detractor to label Klopp's campaign as a disappointing one.

The quadruple was not won. Liverpool, then, join every single club in every season of English professional football in not achieving that particular feat. They simply cannot be mocked for that, even if the tedious nature of social media means many will embarrass themselves by trying.

But while Klopp will undoubtedly be able to point to the size and quality of his squad in an effort to convince many they can go again next season as they aim to snare the biggest prizes in the game, a number of issues have already made themselves known.

Liverpool have their task cut out to effectively replace Sadio Mane

Chief among them, of course, is Sadio Mane's contract situation. Bayern Munich had an offer rejected last week, according to a report on Sunday evening in The Times, and the writing appears to be on the wall as far as the Senegal star's future goes.

A £21 million cash offer with an additional £4 million was rebuffed by the club who are expected to demand almost double that if they are to lose the 30-year-old Mane this summer.

Should he depart, Mane will be the first player Liverpool have reluctantly lost since Philippe Coutinho in 2018 and that £142 million sum received from Barcelona certainly soothed those pains.

For Mane, though, the task of replacing his versatility and quality is a daunting one. It presents Liverpool with a challenge they have not faced for many years as far as the summer window is concerned.

Since Klopp came into the club, the Reds have been operating from a position of strength, adding to the squad, improving the ranks and offloading players who were generally surplus to requirements.

That will not be the case this summer if Mane does eventually depart. Klopp and his recruitment team will be attempting to plug a gap they do not wish to, immediately putting them on the back foot.

Every summer since 2016, which was Klopp's first, has always been about improvement. From the arrivals of Mane, Joel Matip and Gini Wijnaldum, to a year later when Mohamed Salah, Alex Oxlade-Chamberlain and Andy Robertson arrived, right the way through to 2018 (Fabinho, Naby Keita, Alisson Becker and Xherdan Shaqiri), 2020 (Diogo Jota and Thiago Alcantara) and 2021 (Ibrahima Konate) - progress has always been the name of the game.

Each summer window has allowed Klopp and his staff to build the squad further, remove the unwanted, use those funds and continue to put their best foot forward.

This year presents a slightly different challenge then. One that the club must absolutely call to perfection just to stay in the position they are in. The Reds' recruitment team are generally known to search for players between 21-24 who can spend their peak years at Anfield, but the addition of Thiago in 2020 is proof that plans can change if a certain type of profile becomes available. Opportunism is never dismissed out of hand.

Recent history has shown that Liverpool are nimble enough to react to the fluidity of the market forces, at least, so there is no reason for the supporters to be in a state of panic just yet. But there is work to do and once again they must get it right.