I think the back 4 will be Kelly, Toure, Agger, and Enrique. Think Sakho won't be starting this one, even though he's been solid.
Fullbacks are so important in this system. Southampton had 5 players in our half of the pitch when we tried playing the ball out from the back. I watched parts of the game again and it was really astonishing how they totally pressed us up against us, especially at Anfield. It's something we're not really used to at home, and usually when we do lose it's a smash and grab by someone. Sakho and Toure's natural tendency isn't to push up the pitch as fullbacks, and Southampton took perfect advantage of this. With Kelly and Enrique at least we'll have that width to try and push Sunderland back a bit.
The outfield players will be the same against United. Don't think Rodgers will select anything different for a while, especially with only a game on the weekend now. Which of course means no Alberto to start. I think we might see him get the nod against Palace, but not here away against Sunderland.