Here we go again.

Real Madrid v Manchester City in a Champions League knockout match. It is becoming a classic rivalry of our times.

This will be the fourth time the football powerhouses have met in Europe in the last four seasons - but with one big difference.

This time, rather than being in the latter stages, both sides are fighting for their lives to merely make the last 16.

So how did two of Europe's most successful sides in recent seasons fail to make the top eight and end up in a play-off?

Manchester City only limped into the play-offs with a last-gasp victory in their must-win final match against Club Brugge.

A problem of their own making? Yes and no.

Manager Pep Guardiola had the chance to strengthen his squad at the end of the last Premier League title-winning season, but decided against it partly because he was unsure of his own future.

He seriously considered the interest from the English FA and, for a while, he was 70-30 in favour of becoming the next national team coach, with those around him assuring him it would bring him a quieter life.

He also trusted in his players - the squad who brought him four Premier League titles - and ultimately decided he still had a job to do at Manchester City

In November, City were top of the table and the only team in the top five leagues to be unbeaten. Then the wheels fell off.

When Ilkay Gundogan returned in August, the original plan had been for him and Bernardo Silva to play about one out of every three games.

As things stand, Gundogan is just a couple of games away from playing 50% of the matches, which will mean an automatic renewal for him.

Rodri rupturing his cruciate knee ligament and injuries to potential replacements like Manuel Akanji and John Stones has been a key issue.

An out-of-sorts Phil Foden, Kevin de Bruyne struggling to maintain his customary high level and Jack Grealish being unable to demonstrate what a great player he could be, have exacerbated the problem.

The reality was there for all to see as a squad lacking energy laboured and went through the motions in training.

Any remaining uncertainty over Guardiola's City future was ended in the middle of December when he signed a one-year contract extension to take his stay at the club to 10 years.

Against his own better judgement and contrary to the advice being offered by many of those closest to him, he decided now was not the time to leave despite the fact it was becoming clear a total recycling of his squad was needed.

The biggest names in the squad told him they were all with him and trusted he would take them out of the situation.

His authority was never questioned, even though some players have imagined their careers elsewhere.

Guardiola is also battling his own body. He is low in natural defences because of the excessive demands he places upon himself and, with everything in his life at the moment seeming to lack stability, he appears to be picking up every virus circulating.

He is not at his best and he is a very emotional person, but still he is influencing and being demanding to those around him.

Just as things looked to be on the mend on the field, they were put to the sword by a revitalised Paris St-Germain in their penultimate league phase game.

Then, after wins against Chelsea and Club Brugge, they were routed 5-1 at Arsenal. That is how people inside the club expect the rest of the season will be; hit and miss.

With Ruben Dias, Akanji, Josko Gvardiol, Stones and Nathan Ake waiting in the wings, Guardiola is hoping his side can soon once again show the form that earned them a Treble.

At the moment, though, a top-four finish would seem like a major triumph, as Guardiola sets about recycling further elements of his squad in addition to those already brought in.

The coaching staff have already heard their manager say that finishing in the top four would have to be celebrated almost as a fifth consecutive title - he feels it has been that hard.

Real Madrid's route to this unexpected knockout game was not quite as laden with jeopardy as City's, but it has not been without its moments of drama.

Three defeats in the group phase have left them with a game against a side they would normally expect to play in the latter stages.

It will not have escaped anyone's attention - particularly Manchester City's - that despite sitting on top of La Liga, this is a team that has fallen short whenever they have faced a major side at home or abroad this season.

Defensively it has been carnage, with serious cruciate ligament injuries to major players such as Dani Carvajal and Eder Militao, long-term problems with David Alaba and a lengthy absence for Antonio Rudiger.

But defensive problems are only really the tip of the iceberg - as coach Carlo Ancelotti knows better than anyone.

They go into the match against City with a front three of Vinicius Jr, Rodrygo and Kylian Mbappe plus Jude Bellingham behind.

They have amassed 59 goals this season, but the trouble for Ancelotti is his superstars up front - Bellingham apart - have not always put in the necessary shift defensively.

Real's wily boss saw potential problems at the start of the season and knew that unless they became more committed to the cause, it was going to be a long season.

The absence of Toni Kroos has made it more difficult for them to have control of games, with Real suffering from not replacing the retired midfielder.

Any money they have spent has not been on replacing Kroos, but rather on Mbappe and other young, precocious talents.

They have also spent a lot of money on rebuilding their stadium, while complaints from the neighbours about noise means that concerts planned to take place at the new ground have been cancelled by the local licensing authorities.

To say this was a 'nice little earner' for the club does not even begin to describe the situation. This is a big financial blow to Real.

Vinicius Jr looked far more committed to the cause in Real's match against Atletico at the weekend, having been read the riot act by Luka Modric and Federico Valverde for failing to give his best during the club's quarter-final Copa del Rey match at Leganes.

Ancelotti continues to attempt to deal with a player who often looks like he is trying to take on the world, both on and off the pitch.

He appears to be carrying a heavy burden, which means we have not seen the best of him this season.

Lurking in the background is also the reported massive offer - some say about 1bn euros - from Saudi Arabia for him.

At the moment it is all talk and Real Madrid are leaving the door open without actually saying anything for or against the deal.

Although, with Rodrygo having scored 13 goals from Vinicius' favoured left side, his departure would not be considered a fatal blow.

Mbappe, meanwhile, is the best he has been so far at the club.

He has decided to rely on his own skill and intuition and on being himself, and now everyone - Vinicius included - knows he is the focal point for Madrid.

The leadership pecking order is beginning to take shape, led by Mbappe and a much sharper Bellingham. Modric and Valverde also have a say, replacing the likes of Carvajal and Militao, who were at the forefront.

Describing Real Madrid as exposed seems something of an understatement. But as Ancelotti says, they have been winning with quality, if not commitment, and that is in abundance.

So who's going to bet against them?