From the FAQ
(144856.43):
“The injury said it was for 1 week but it has been more than 1 week. Why?”
This is because the time is an estimate. It is just like the arena construction estimates and can often be shorter or longer than first estimated. Another reason might be because one week can mean anything from 1 day to 13 days and so although it says 1 week, this is because it has been rounded to 1 week.
An injury can also be shorter because you have a doctor that is helping with healing speed, although the effect that a doctor has on the healing of a player is less significant than the initial reduction in severity. That initial reduction is based on the doctor you have on staff when the injury occurs.