That's true, that's why this is a free-browser game and cannot be compared to PC/Console manager-kind purchasing games.
Nonetheless, my suggestion is just "aesthetic", since it doesn't affect the engine of the game. At this moment, as the cap for players in a team is 22, that's the highest numer a player can have, so why not make it possible that I can change it as simple as I can change his position in the field?
It's not a big deal, nobody will be able to do "weird things" like giving a 69 number to a player, and it doesn't affect at all the way of the game.
Having to pay 1T to change the name and nationality of a player? I understand that, that's a luxury... but choosing the numbers of the players its part of the Manager duty.
Just a suggestion I'd like to see introduced... I'm not going to pay 1T to change the number of a player, but if I could do it freely I would, and it would be a very simple thing that would make the game more enjoying to me.
Thanks for your attention Mr. Mod!