Since the inception of squad numbers, football has had a strange obsession with the numbers players wear on the backs of their jerseys. Numbers are handed down from player to player, holding a certain prestige, inspiring columns like "the best number 10s of all time" and even books filled with tactical analysis.
Shirt number reveals are even a big thing, with clubs using the announcement as another PR segment in the non-stop world of social media. Alexandre Lacazette took the number nine shirt from Lucas Perez and annoyed his predecessor in the process.
The players themselves have a certain connection to certain numbers, and like to keep them from season-to-season, and even when they move to a new club. Nemanja Matic's £40 million transfer from Chelsea to Manchester United did not allow him to retain his favoured 21, with fellow midfielder Ander Herrera already sporting those digits at Old Trafford.
"Nemanja is a Manchester United player and a Jose Mourinho player. He represents everything we want in a footballer; loyalty, consistency, ambition, team player," Mourinho told the club's official website.
"I would like to thank him for his desire to join us because without that, it would be impossible to have him here. I am sure our players and supporters will love him. A big welcome to our new number 31."
Matic inherits the number from Bastian Schweinsteiger and before that, Marouane Fellaini - and fans will be hoping the 46th most expensive signing of all time can fare much better than the last two players to run out with that number on their backs.
Schweinsteiger and Matic both share the same birthday - August 1 - with just four years separating them. The 33-year-old German was allowed to leave United for free to Chicago Fire, having played no league games and just four matches overall last season. One is worth nothing to Mourinho, the other £40m.
Matic will compete with fellow 29-year-old Fellaini for a starting position at Old Trafford, and with the Belgian now staying at the club despite links with a move to Galatasaray, his new Serbian rival will have to prove his value to Mourinho and Manchester United.
Back in 2015, when both were at Chelsea, Mourinho sent Matic on at half-time against Southampton only to withdraw him 27 minutes later after the Blues had conceded twice. Under Mourinho's successors, he remained a regular in the Chelsea XI, and started 30 games as they won the Premier League last season. In comparison, Fellaini started just 18 for Mourinho, coming off the bench in a further 10. But is Matic really an upgrade?
When Mourinho took him off after just 27 minutes, he had no regrets. Never one to shy away from criticising his own players, the Portuguese took aim after the match, telling reporters: "Some of [the players] are in a difficult moment, Matic is one of them. He is not playing well. He is not the sharpest. He is not making good decisions with the ball."
There is no doubt that Man Utd needed a top holding midfielder who can shield the back four and give the likes of Paul Pogba the licence to attack. His power, height and experience will certainly be an asset.
But the truth is that for 18 of the last 24 months, Matic has looked like a player in decline. He had a poor 2015-16 season and despite a good first half to last term under Antonio Conte, he again declined badly after Christmas - looking slow and lumbering as Cesc Fabregas increasingly gained more faith from his manager.
Even when Mourinho was singling him out for criticism, he made 129 tackles in the Portuguese manager's final Chelsea campaign, but contributed less than half that amount under Conte. In less games, Fellaini had 56 to Matic's 51 tackles.
Their passing percentages of 87.72 for Matic and 86.01 for Fellaini differed little, while the Belgian averaged 0.66 passes per minute to Matic's 0.69. They also scored the same amount of goals, with Matic only bettering Fellaini in the assists and interceptions columns - although the latter was occasionally used higher up the pitch by Mourinho last term.
It was noticeable by the end of last season that Conte no longer considered him an important player in his team - and it is telling that the Italian was more than happy to sell the player to a domestic rival.
The last player with 31 on his back, Schweinsteiger was deemed as surplus to requirements when he was just 32-years-old, having looked a shadow of the World Cup winner that United signed from Bayern Munich. The new No. 31, Matic is just three years younger now, and will need to hit the ground running at Old Trafford to prove he is worth the £40m outlay and not just there to make up the numbers.