Didi was only one problem, Taylor was the other.
Let me explain the hypocrisy:
Back in the 90's Scott Fisher ruptured his ACL, ruling him out for roughly a year. In the following season, Perth signed an injury replacement. As Fisher was almost ready to return, Crawford suffered a minor, but season-ending injury. The Cats released their injury replacement, reinstated Fisher, then immediately re-signed the same player as injury replacement for Crawford.
For some reason this created such an outcry, that the league changed the rules, although it still would have allowed that scenario.
Basically the rule says that once you release a player, you can only re-sign them as an injury replacement for a season-ending injury.
The purpose of this rule is precisely to prevent what Sydney did. Keeping an extra decent player on the payroll, and using them as injury replacement for various players throughout the season.
Like a lot of rules, the league can allow it under special circumstances.
Eg, last season Perth used Sunday Dech as a replacement at different times.
But once again the "rules" have been further stretched to accommodate Sydney.
Can you imagine if Cotton naturalises, and next year Perth were to have an extra import hanging around all season just to be an injury replacement? This place would go into meltdown.