Exactly how does it work?

 As I understand E5 went to AA FisherCat Rehab last week during FisherCat Playoff's game before coming back to the Major, I got couple questions, please help me out...

1. Does rehab basically sending a player(player A) down from DL to the Minor and has to send somebody else(player B) down to minor or DL before the player(player A) will be able to come back to the major?  Does the player(player A) has to clear the wavier when send them down to the minor for rehab?

2. I suppose not every player needs to go through rehab before playing in the major, assuming if a player got injuried during september and got better before the playoffs yet all the minors season's over, is that mean there is no way for them to 'rehab' before coming back to the major?

Let me know.  Thanks.