Once  you are down on the ground do you stay there until the end of the week?

 Yes.  You might get injured and taken out.  You might get some special card like an Apothecary that lets you get a guy back up again.  But normally... yes you just stay down.

I had found the Injuring Rules and Apothecary right after posting this. :)

on the injuries rule, when i tackle a stronger player (or my dice) comes up as tackler/attacker down ie my player is downed, is my player injuried right away?

i'm a little confused on the wording (pg.11, left colum "Tackler Down")-

"If the tackling player is standing, he becomes downed" Surely my tackler has to be standing to make that move??

and

"In the rare situation that the tackling player is downed, he becomes injured" So if my dice come up tackler down he goes straight to injured everytime?

No, both lines say exactly what they mean. I think, from your comment on the first line, that you are making assumptions on what is possible within the game and so merging the rules into one

The first is the basic, common situation where the tackling player is standing and says that tackler down result puts you down. This will be the rule you follow in 90%+ of tackle attempts.

The second line describes those rare occasions when you can get a tackling player who is already downed - mainly its card plays that lets a player tackle while down (the dwarves have one for example, the tackling coach card could do it for anyone). There is even a chaos player who is better tackling while down as their star player rating goes up so chaos players may try and engineer the effect. In those rare cases the rules are telling you that if you take a tackler down result on an already downed player, you become injured.