
Yes, it’s conceited to draw attention to this, but we were the first to flag the possibility of Henrikh Mkhitaryan wearing two different shirt numbers for Arsenal this season:
While it seems like news to the many clickbait outlets who have referred to an “obscure rule”, UEFA have long had it in their regulations that a club can’t have a number worn by two different players in the one season.
For various reasons, the phenomenon of players wearing a different number in international competition compared to that used domestically goes all the way back to 1996-97, when UEFA allowed teams to use squad numbers (they had been in force in the Premier League since 1993-94 and Serie A, La Liga and the Bundesliga since 1995-96).
Initially, UEFA decided to go the same way as the Spanish league had, by insisting clubs numbered their squads 1-25, with 26 and 27 empty if they so wished for mid-season signings and 28 upwards used for B-list players (generally, home-grown youngsters).
At Manchester United, one of those B-list players was Philip Neville, who had switched from 23 to 12 in the summer of 1996, but wore 28 in Europe.

Taking Neville’s number 12 was Paul Scholes, with 18 worn by Chris Casper, who was 26 domestically. Another change was that Ronny Johnsen, given 19 on his arrival, wore 5 in Europe as that had been freed up by the departure of Lee Sharpe. From 1997-98 onwards, Johnsen wore 5 in all competitions.

In the treble season of 1998-99, Wes Brown wore 30 for United in the Champions League while he had 24 as they won the double.
Back to 1996-97 and changes for Arsenal saw Matthew Rose switch from 31 to 22 for Europe and Paul Shaw going from 27 to 23.
Newcastle United had a few notable differences, the most prominent being Philippe Albert, who had to swap his 27 for 12 (far right of right-hand pic, apologies for the lack of a better 27 image).

Other changes apart from Albert were Robbie Elliott (26 to 12), Paul Kitson (28 to 22) and Paul Barrett (31 to 29).
A strange change came for Arsenal in 1998-99. Ian Wright left the summer after the club won the domestic double, freeing up 8. It was eventually taken by Fredrik Ljungberg, but he arrived too late to be registered for the Champions League. Instead, in Europe the shirt was assigned to Michael Black, who wore 28 in the league. Black’s only appearance was as a late sub in the last group game, a dead rubber against Panathinaikos, hence the poor quality of the picture.

Stranger was the following season, where Matthew Upson – who had 20 as his ‘normal’ squad number during the entirety of his Arsenal career – had 29 in the Champions League.
In that 1999-2000 campaign, 20 was worn in Europe by Stephen Hughes – he had had 16 in 1998-99 (and in the UEFA Cup in 1997-98 while still 28 in the league) but then seemed set to depart and so 16 was given to Silvinho. When Hughes returned from a loan spell with Fulham, he was re-assigned as 23 domestically.
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Also in 1999-2000, Manchester United took part in the first staging of the Club World Championship, where squads were limited to 23 players and numbered consecutively. With David May not involved, his number 4 was taken by Danny Higginbotham, usually 28; back-up goalkeeper Paul Rachubka was given 13 but didn’t have a number for other competitions; the other changes were Mickael Silvestre (27 to 5), Jonathan Greening (34 to 15), Mark Wilson (33 to 18), Quinton Fortune (25 to 22) and Ronnie Wallwork (30 to 23).
Real Madrid also took part in that inagural event. While the 1-25 rule in Spain meant that there weren’t too many changes, they did a few alterations. Goalkeeper Bodo Illgner had fallen out of favour so Albani Bizzarri swapped 13 for 1 with Iker Casillas dropping from 27 to 13 and third-choice Oliver changing from 28 to 23. The other difference was that number 20 Elvir Baljic didn’t travel so Perica Ognjenović changed to that from 25.
The 2005 Club World Championship was the last to limit shirt numbers. In practice, the biggest shake-up was for Liverpool goalkeper Pepe Reina, who usually wore 25, but now donned 12.

Another player forced to come down was number 24, Florent Sinama-Pongolle, who wore 11, while Scott Carson, 20 at home, switched to 13.
The remaining three differences were only paper changes, as none of Danny O’Donnell (5, no domestic number), Darren Potter (18, from 34) and David Raven (20, from 31) actually travelled to Japan for the competition.
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The rule which governs the assignment of numbers reads thus:

The part about no number being used by more than one player is what is stymieing Mkhitaryan, but it is the second clause which worked against Lionel Messi in the 2005-06 season.
He began the campaign with the same number 30 as he had used in 2004-05 – as a younger player, he didn’t have to be given a lower number – but he became such an integral part of the team that, during the winter break, Barça took advantage of the ability to change a player’s number by giving him the number 19.
The image on the left is from Barcelona’s league game away to Mallorca in January 2006; the one on the right is from the Champions League tie at Chelsea in February, with the switch not having been possible for Europe.

Lassana Diarra has recently signed for Paris St-Germain – it’s the midfielder’s tenth club so it’s hardly surprising that he has at least one example of being forced to wear two different numbers in the same season.
He joined Real Madrid from Portsmouth in the 2008-09 winter transfer window. With his unrelated namesake Mahamadou Diarra ruled out for the remainder of the season with injury, Real re-assigned the number 6 to the new signing, but rather than wear the exact same shirt with ‘Diarra’ on the back, he opted for ‘Lass’.

Of course, he couldn’t wear 6 in the Champions League and so had 39 on his back as Real crashed out against Liverpool – quite possibly the highest shirt number used by the club in a first-team game.

That same 2008-09 window saw Pascal Chimbonda return to Tottenham Hotspur after a short spell with Sunderland. The number 2 that he had worn in 2006-07 and 2007-08 was now in the possession of Alan Hutton, so Chimbonda opted for 21 – however, as that had just been vacated by goalkeeper César Sánchez, Chimbonda had to have an alternative for the UEFA Cup.
He went for 97. It’s probably best that we don’t have a picture.
Other Tottenham players to have had to go with dual numbers are Lewis Holtby in 2012-13 (23 in the league, 14 in Europe) and Erik Lamela and Vlad Chiriches in 2013-14.
That pair were both signed before the end of the summer transfer window. Lamela had 33 as he sat on the bench for the game against Arsenal but, as Gareth Bale was sold to Real Madrid days later, he was able to switch to 11 – only domestically of course, keeping 33 in Europe.
Scott Parker had been switched from 8 to 6 during the summer but was then sold to Fulham and, as he was part of the provisional European squad, it meant that Chiriches took 12 for the Europa League while wearing 6 at home.
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The final example is quite similar to the situation which inspired the article, that of the transfers of Mkhitaryan and Alexis Sánchez. Because Manchester United’s number 7 has been empty all season, Sánchez can wear it in the Champions League, the same way as Fernando Torres was free to wear 9 for Chelsea in all competitions in 2010-11.
The man who took Torres’ number 9 at Liverpool that season, Andy Carroll, wasn’t so lucky, though. When the Reds were eliminated from the Europa League by Portuguese side Braga, he was attired in the number 29.
