Manchester City are among the elite European teams investigating the possibility of making arguably the biggest signing of all time in Lionel Messi, but there is one question it seems almost sacrilegious to ask...

The Argentine, who has told Barcelona he wants to leave, is undoubtedly one of the greatest players ever - perhaps even the best - and one of the most successful too.

And while the 33-year-old was the top scorer in La Liga last season, it was still his worst campaign for goals and appearances since 2007-08.

And yet, a record-breaking season for assists tells another story entirely.

So BBC Sport examines whether Messi is in decline - or if he is still the greatest player in the world.

Show us your medals

The reason it seems remiss to ask whether Messi is worth signing for a fortune is because of what he has done in his career.

Messi has won more Ballon d'Or awards - given to the best player in the world - than anybody else, with six. That includes the most recent one in December 2019.

He is Barcelona's all-time top scorer with 634 goals in 731 games, and La Liga's record goalscorer with 444 in 485 appearances.

His personal peak was probably in 2012 when he scored 91 goals for Barcelona and Argentina in 69 games.

He has 34 club honours, including 10 La Liga titles and four Champions Leagues.

On top of that he is Argentina's record scorer with 70 goals in 138 international games.

"He is simply the greatest to have ever played the game," said former Barcelona striker Gary Lineker in a BBC Sport column in May.

"An extraordinary combination of things makes him a complete one-off. Messi is also right up there with Diego Maradona as the best dribbler there has been. On top of that, he's also quite possibly the best passer of the ball we have ever seen."

Is he still the best goalscorer in the world?

"Messi might not be quite as good as he used to be, but his performances last season were enough to show he is still the best attacking player in the world - by far," said Spanish football journalist Andy West, who has written a book on Messi.

But on goals alone, he is not the best any more.

Messi won a record seventh Pichichi (La Liga's 'golden boot') - his fourth in a row - with 25 goals, four clear of Real Madrid striker Karim Benzema.

That was despite playing in the worst Barcelona team in years, the first to finish without a trophy since 2013-14.

It was the first time La Liga's top scorer managed fewer than 30 goals since Mallorca's Dani Guiza 12 years ago. Deportivo la Coruna's Diego Tristan, in 2001-02, was the last player to win it with fewer goals than Messi.

Yet he still made two bits of history (three including becoming the first player to win seven Pichichis).

Messi broke Xavi's record for assists (20, 2008-09) in a La Liga season by setting up 21 goals.

He also became the first player to score and assist 20 goals or more in a La Liga season. The last player to do that in Europe's top five leagues was Thierry Henry at Arsenal (24 goals, 20 assists in 2002-03).

Of all the players in Europe's top five leagues (in all competitions) Messi was only the joint-sixth top scorer - with 31 goals in 44 games (0.7 goals per game).

Bayern Munich's Robert Lewandowski scored 55, with Serie A trio Ciro Immobile, Cristiano Ronaldo and Romelu Lukaku as well as RB Leipzig striker Timo Werner, now at Chelsea, all scoring more than Messi. Manchester City's Raheem Sterling also scored 31 goals in all competitions.

But when you factor in assists too - 25 in all competitions - Messi shoots up to second with a combined 56 goal involvements.

Only Treble winner Lewandowski, who would probably have won the Ballon d'Or if there was one this year, is above him.

In all competitions Messi attempted more dribbles than anybody else from Europe's top divisions (403), with only Wolves' Adama Traore - once of Barca - completing more (275 to Messi's 260).

How do Messi's stats compare to previous years?

Messi is showing no real sign of decline in terms of touches, passes, dribbles or chances created - and only a minimal one in terms of shooting.

With help from Opta we have looked at his data since 2008-09 (the season in which he first reached 20 goals) in his main two club competitions, the Champions League and La Liga. The same amount of data is not available in some of Barca's cup competitions.

His 24 assists in those two competitions in 2019-20 is more than he managed before in his career (only passing 20 three times before).

And while 126 minutes per goal was his worst ratio, his 68 minutes per goal or assist was better than his record in four of the previous 11 seasons.

He attempted his fourth-highest amount of dribbles in that time, and had his best success rate (65%) yet.

His passing accuracy was 82%, as high as it has been in four years, although slightly below his 85-87% accuracy for the five years from 2009-10.

It was a similar story for his passes into the final third of the pitch.

How has his positioning changed?

As time has gone on Messi's position on the pitch has got deeper. He starts a lot of his games on the right wing technically, but his play is usually based more deeper and centrally.

Statisticians Statsbomb have calculated where the majority of his attacking touches have come in each of his La Liga campaigns.

Last season many of his touches came centrally behind the striker - although he did get involved more on the right side of midfield than in previous campaigns.

"Messi has lost some pace, and couldn't now beat many opponents in a 20-yard sprint," said West. "But his footwork with the ball is as fast as ever, retaining that magical ability to nip the ball away from defenders just as they think they've tackled him.

"Last season he played deeper than ever before, dropping into midfield to act as a playmaker rather than leading the attacking line.

"That was partly out of necessity because no other Barca player was capable of doing the job, but it looks likely he will spend the rest of his career as a traditional number 10 attacking midfielder (who happens to guarantee 20-plus goals per season) rather than a forward.

"That's a less physically demanding role, allowing him to save his physical and mental energy for the things he still does better than anyone else: passing, dribbling and shooting."

City boss Pep Guardiola, Messi's former Barca manager, put it best when he was asked if Sergio Aguero is the best central striker he has managed.

"The best is Messi," he said.

"Messi is [the best] number nine, number 10, number 11, number seven, number six, number five, number four…"

So is Messi still the best?

There is still a strong argument to say Messi is one of the very best players in the world, although Lewandowski probably had a better 2019-20 season than anybody else.

He is one of the best goalscorers in the world, one of the best dribblers and one of the top passers.

There is also a strong argument that he could do even better in a strong team, with Barcelona looking ragged at stages last season even before their 8-2 season-ending thrashing by Bayern Munich in the Champions League, which looks like it may be Messi's last game for the club.

He won the La Liga 'golden boot' and was involved in more goals than anybody else bar Lewandowski last season, so would clearly still be a huge asset for any team.

The only thing not on his side is time, although former Barca team-mate Xavi recently said he thinks Messi could play until he is 40.

MOTD host Lineker said: "It has been an absolute privilege to watch Messi and his last game will be one of the saddest days in football history.

"No footballer can last forever and you can start to see that his powers are waning a little bit. It doesn't matter how good a player you are - you cannot beat Father Time."

West said: "Messi is still the best, and he will be for a few more years yet."

So what do you think? Is 33-year-old Messi worth buying at all costs?

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