I agree with
TNJAKE
in that injection is a tool, not a rule. I inject all briskets, usually with a weaker version of a competition style injection from Kosmos or Big Poppa Smokers.... but my old standby recipe still works and it's roughly 50:50 mix of beef soup base (or thinned down beef consume) and beef aujus (any of the paste or powder mixes work fine, just adjust to your tastes.) I shoot for 1 ounce per pound and will pour any leftover into the bag with the brisket. This mixture will be a little salty, but consider the brisket will be cooking for 8, 10, 12 hours and the flavors do meld with the beef.
There is one trick called a 'late injection', and you need some savory au jus, not too salty. Maybe 4 ounces or so. I do add a shake or two of Worcestershire to this. When your brisket is in the stall warm up this injection, and shoot the brisket in an even pattern. All you are doing here is giving the meat a flavor boost, and adding moisture.
If you like a more complex au jus, here is the recipe I serve with prime rib and it freezes well.
RRP AuJus - ~thirdeye~ version
(My version begins with beef consume for a deeper beefy flavor, uses dry onion soup mix, and more water as the consume is saltier and thicker than beef broth. I went heavier on Wooster and added soy sauce to increase the umami*, and added a couple of other ingredients).
1 14 oz can Beef Consume
1 packet Lipton Beefy Onion Soup (dry mix)
1-1/2 of the soup can cold water
5 teaspoons Worcestershire sauce
2 splashes of Soy Sauce
1 teaspoon of garlic salt
2 teaspoon Herb-OX beef bullion (or other trusted brand)
2 teaspoons Montreal Steak rub
1 teaspoon Smokin’ Guns Rub (or your favorite rub)
1/2 teaspoon celery seed
1/2 teaspoon black pepper
Bring ingredients to a low simmer in a saucepan, and cook for 15 to 20 minutes. Try to avoid having the liquid come to a boil. Additional water can be added if flavors are too strong, or if liquid gets too thick. AuJus is a thin table sauce, NOT a gravy. Pour through strainer to remove the onions and coarse pieces from the Montreal Steak rub. Serve warm