Most players refuse and get ****** off if you want to give them an injection whenever I have tried it. Just have a bigger squad to replace the injured player and let the physio treat him.
I've only given one player an injection before a game. His fitness was so poor I didn't even end up playing him though. So for all injuries I send them to physio.
I've probably given 10-12 injections on FM10. Usually has happened when I'm new to the club and haven't had time to add depth to the team. And only to the older players and if I decide to use him then only as a sub.
i have only ever given a injection to a player once and it ended up in them having fitness problems for 2 years and then he retired, i think he was only 28.