When Alex McLeish was the manager of Rangers in 2002, his son Jon told him Barcelona had a kid on the fringes of their C team who was going to be the best footballer in the world.

Jon knew this because he played the computer game Football Manager (FM) and it suggested the 15-year-old player was brilliant.

McLeish didn't sign the boy in question but history has shown Lionel Messi turned out OK. It has shown the mechanics underpinning FM are far from guesswork, too.

Football Manager was aware of Lionel Messi's talent when he was coming through the Barcelona ranks

Jon McLeish also noted Andres Iniesta's potential via the game. His father agreed a loan deal but then Iniesta was needed by Barca's first team and never made it to Ibrox.

'Every manager worth his salt will be using it,' McLeish says of FM, these days propelled by the largest collection of real-world, in-depth player data, bar none.

No club, no association, no analytics firm, not even FIFA has amassed a cache like the one compiled over 20 years by Sports Interactive Games (SIG), who own FM.

From an office block HQ near Old Street Tube station in east London, SIG employ more than 1,200 scouts across the world to gather intelligence. Their database has details of 650,952 people. Precisely 325,429 are currently active as players, managers, coaches or support staff at thousands of clubs in hundreds of divisions in dozens of countries. The majority of the rest are ex-players.

Former Rangers boss Alex McLeish says his son told him about the Argentinian when he was 15

It is such a treasure trove that ProZone use it as part of their 'Recruiter' software. Clubs use that to pick targets for scouting, and possible signings. Child's play? Far from it.

Miles Jacobson is the SIG boss, or 'studio director'. He's a former NME writer turned music biz executive who worked with Blur, the Bluetones and others in Britpop's heyday. Almost 14 years ago he was hired by Paul and Oliver Collyer — the Shropshire brothers who created FM in their bedroom in the early 1990s — and has helped transform their game into a serious football brand.

'They always had the vision of making a living, breathing football world,' Jacobson says. 'They wanted clubs that were fully formed, from the youth sides up.'

Messi soon broke into the Barcelona first team and is now one of the world's greatest-ever players

Real football players and managers use it for business and pleasure. France striker Bafetimbi Gomis used it to gauge the quality of Swansea's players before he moved there from Lyon in 2014. His international team-mate Paul Pogba was spotted on the team plane buying himself - while playing as Chelsea's manager - the same year, sparking rumours he wanted to go to Stamford Bridge.

Tottenham and England winger Andros Townsend recently got in hot water with his girlfriend when a club statement about a suspension for being late to training was posted on Twitter. He had to explain to her somebody had screen-grabbed this fiction from FM.

Former Manchester United striker and Cardiff manager Ole Gunnar Solskjaer, 42, now back in charge at Molde, has played FM for more than 20 years. 'I've seen people I bought and sold as teenagers [in the game] become big players in real life,' he says.

Juventus midfielder Paul Pogba was pictured playing the game as Chelsea manager

The Mail on Sunday spent an afternoon last week with Jacobson, a Watford fan, and his key staff, 105 of whom are based at FM's HQ. Jacobson describes a Venn diagram depicting football obsessives in one circle and coding geeks in another. 'The bit where those circles intersect is the people who work here,' he smiles.

As a special exercise, FM's analysts have used database metrics to select 11 players currently seen as attractive transfer targets this January.

Jacobson's enthusiasm is infectious, his workplace a shrine to the game (football) and the game, FM. His office is crammed with memorabilia, signed Messi and Cristiano Ronaldo shirts here, the last Manchester United jersey to be signed by both Alex Ferguson and Roy Keane there.

The Astroturf pitch across the road is routinely filled with FM staff playing in their lunchtime five-a-side league. Jacobson is proud of his company's affiliation to 'real football'.

SIG has been the shirt sponsors of AFC Wimbledon since that club's genesis in the wake of the FA's acrimonious decision to allow the formation of 'franchise' side MK Dons. SIG's deal with AFC Wimbledon, at almost 14 years, is one of the longest continuous shirt sponsorships in English football.

Andros Townsend's girlfriend believed the star was fined after fan sent her a Football Manager screenshot

And Jacobson is always looking to enrich FM's authenticity. The first database in the early Noughties had 4,000 players, with Roma's 39-year-old Francesco Totti the only player active in it both then and now. Originally there were only 20 'metrics' per player, rating their passing, heading, shooting and so on.

Now there are up to 250 metrics per person, or some 100 million bits of information, gathered by around 1,200 researchers working under 70 national and regional heads of research, in turn answering to seven research chiefs at HQ.

Former Cardiff City boss Ole Gunnar Solskjaer is also an avid fan and has played the game for years

The 250 items now go way beyond technical ability, on-pitch strengths and biographical basics like age, height, weight, appearances, goals and honours. Passport eligibility is there, and languages spoken, and contract details, and salaries when attainable, which is more often than you'd maybe imagine, especially at smaller clubs where scouts have good relationships.

There are multiple 'preferences', of playing position, future clubs or professional ambitions, or anything, when stated in interviews or biographies or shared with scouts.

Also there in the database are an array of metrics never overtly made known to those who play FM but impact on the way it unfolds. Temperament, discipline, proneness to being injured, and indeed injuring others, are all there. These are not 'public facing' facets for legal reasons; imagine the outcry if FM declared a player liable to be dirty or a crock. Yet all that real-world information is swirling in the currents that drive the game.

Roma star Francesco Totti is the only active player from Football Manager's 'Noughties' database

Other legal issues cause headaches, such as a need for licences from many unions and leagues to use player names. In some cases these are unattainable. The names of players at German clubs can't be used, for example, following an intellectual property court victory by Germany goalkeeper Oliver Kahn over another games manufacturer in 2003. FM is not sold in Germany. And players under 16 now cannot be named for data protection reasons.

FM sells around a million 'hard' copies per year, half in the UK, then next most in France, Scandinavia, Italy, Brazil and the USA. The same amount of players again play mobile versions, and online versions, and several times more again play pirated copies.

The typical user will spend more than 200 hours playing each edition. It's not unusual for them to turn up unannounced at HQ, as one did last week, from Hong Kong. He walked past security on to the editorial floors, enthused on FM to staff, and then took them to task about improvements required in the game.

Or rather a simulation: as McLeish and Messi — now a five-time Ballon d'Or winner — might attest, FM is more than a game.