Everyone earns his place in the starting eleven. You don't drop Skrtel for Agger if Skrtel does not warrant dropping.
Not to clog up the Sakho thread, but that's nonsense. Even if Aspas was setting the world alight during the first few games he would have been dropped for Luis Suarez (and rightly so). Every time Gerrard goes out injured he comes right back into the team when he's healthy, even if his replacement had not "warranted dropping" (again, rightly so). The idea that players can be "on form" is a complete and utter fallacy - it's a mistake that people who don't understand probabilities make (the same way that the "hot hand" in basketball is). Simply put, you play your best players - you don't think "well, Player X hasn't screwed anything up in the last three games, so I'll wait until he does to bring someone else in." That would be the height of lunacy. It's also worth pointing out that if you set up a team structure where if a player is injured they lose their place in the team, then you encourage players to play hurt, which then damages the team.
So in the end, if BR thinks that Agger is the 4th best CB on the team and thus plays Toure/Skrtel/Sakho, so be it. That's a decision based upon some rational analysis. If on the other hand he thinks, "Agger isn't the 4th best CB on the team, but I can't drop one of the players Agger is better than until that player does something disastrous" - well, honestly I can't fathom anyone who has gotten to BR's level thinking something so completely and totally irrational...
Returning to Sakho. I think he's going to be great. I also think he's going to cost us points this year because he doesn't yet know the difference between being aggressive and being stupidly aggressive. If Crystal Palace weren't so wretched they would have punished us on the play that Sakho went clattering into Skrtel for no damn reason (the ball was Skrtel's and also offered no threat) thus springing Palace onto a break (in exactly the same way he did against Swansea where we were actually punished). But as I've said previously, I think it's easier to get a naturally aggressive player to be a bit smarter than it is to get a non-aggressive player to be aggressive...