Why TNS player had shirt number written in pen

The New Saints staff had to write on the back of a replacement jersey for midfielder Rory Holden using a ballpoint pen during their Champions League qualifier with Shkendija.

Why TNS player had shirt number written in pen
The New Saints staff had to write on the back of a replacement jersey for midfielder Rory Holden using a ballpoint pen during their Champions League qualifier with Shkendija.