Retired the non-retired?

Will someone explain this to me: How can you retire a player's jersey if that player himself hasn't filed for retirement yet?
If Brett Favre decides to play one more year this winter, there could be a couple interesting storylines developing.
If he stays with the Packers we could have the unprecedented scenario of a player having his jersey retired while leading his team to victory on the same night. No doubt that Favre would pass for numbers close to 350 yards, three touchdowns, and of course, three interceptions. The final touchdown would probably come in the last two minutes to give them a victory against the Vikings.
The second possible scenario is that if he gets traded to another team, and has to play in another city on the same night that his jersey is supposed to retired. Favre is so amazing that he could probably put up the same numbers for another team and still attend his jersey retirement simultaneously.