No one will deny that shirt numbers should be clearly legible. But how can legiblity be reached? The UEFA has a Kit Regulation for international European football games.
In practice this regulation has a woeful consequence: stripes on shirts are judged to harm legibility in such a way that UEFA forces teams to print numbers on a neutral-coloured patch. In most cases the typical pattern of the shirt is disturbed.
Looking again at the kit regulation, I can't but conclude that one important means for improving legibility is missing: letter shape. As pointed out in the theory section, legibility is effected by for instance the construction and thickness of the figures. In pratice all kind of shapes are allowed by the UEFA, as long as one can see which figure is meant. Sometimes this leads to very strange figures that can only be identified because one knows that it must be a number. When such a number is moving and influenced by the folding of the cloth, legibility diminishes rapidly.
In my opinion patches on striped shirts are often ugly, so they should be avoided (see: stripes).
Stripes with a low light-dark contrast barely harm legibility of shirt numbers. Applying a plain patch doesn't really improve legibility and it spoils the shirt design.