The injury may end his football career.
His football career is over, due to injury.
The term 'for good' is in use as an alternative for 'forever' when speaking about something that is ending. It has a suggestive meaning it is for the best that thing is ending. My grandma uses the term with venomous accuracy, but young people seem to use it little.
forever vs always. They are synonyms. While they mean the same thing always is an absolute. Always reaches infinitely into the past and the future, while forever has the sense of infinity into the future from this point on. Forever enjoys colloquial versatility where always does not. It is wise to avoid always unless it is true.