Sakho just turned 24, and he's certainly got more technical ability than Chris Smalling, or a lot of other decent CB's. I don't think that was a bad signing at all.
I just watched the Liverpool goals again, and Goal 1 is basically just Gerrard's fault (he doesn't mark Delaney and instead stands around doing nothing, because he's not a DM is he, he just plays deep now. Esteban Cambiasso and Claude Makelele are rolling over in their graves). Sakho is marking his man and in the correct position.
Goal 2 is the break after the corner, and it's definitely mostly Johnson's fault for getting run around too easily by Bolasie, failing to just foul him and take the yellow at the half-way line and then being too scared to get closer to him once he's in the box for fear of getting a pen (this part a bit more understandable, since Bolasie really is a fantastic dribbler). Flanagan does a poor job marking Gayle too, and is definitely partly at fault. Sakho jogs back (and Gerrard to his credit sprints back like a madman) and though it's doubtful he gets there, it doesn't look great when a man 10 years old is blowing by you.
Goal 3 is mainly Gerrard and Lucas's co-fault for marking absolutely nobody, instead of one of them dropping deep to help out Skrtl, so when the ball goes over them Skrtl ends up 2 on 1 against Murray and Gayle and instead of dropping off and forcing Murray to chest it down and hit a 20 yard volley for the goal (high degree of difficulty) a simple chest past Skrtl and Gayle is in alone. Sakho is correctly marking Tom Ince, who has gotten inside of Flanagan, who has fallen asleep. As is Glen Johnson, who should have tucked in upon seeing that Lucas and Gerrard have not dropped in and Skrtl is 2 vs 1, especially since there's no left-sided player to threaten him as Ledley is tucked in and deeper.
So, overall and very stupidly, I'd break down Liverpool's collapse like this, including Rodgers's failure to make the right sub (bring in Toure or Agger to play DM once it got to 3-1, and definitely once it got to 3-2!) and stop the bleeding:
Gerrard - 40 percent (instrumental in 2 goals by failing to do basic DM stuff when you're leading, which I do and I'm shit and play DM with the entitlement of Pirlo )
Rodgers - 30 percent (He had to know Gerrard might wander out of position. It's defined his whole career. Also, his team fell asleep on a set piece for the 2nd as he made a sub during it, and it was the wrong sub, too)
Johnson - 15 percent (Mostly for the 2nd goal - it's kind of understandable he got beat by Bolasie, but he did get beat, then failed to just haul him down for the yellow or put the modicum of pressure required to stop Bolasie, a generally horrific passer, from making the perfect pass to Gayle. Also involved in the truly calamitous 3rd goal.)
Lucas - 10 percent (seems high, but he's playing DM with Gerrard and has been for years, and yet doesn't drop in on the 3rd goal (and if you leave someone who habitually wanders away from babies to dress up like Roy of the Rovers and go on week-long benders to babysit for a week and the baby is dead when you get back, it's a bit your fault) and on the 1st, he's neither marking Delaney nor realizes he's behind him, and is instead trying to cut off Jedinak's potential pass backwards to half, which uh, would have been fine)
Flangan - 5 percent (If he stands closer to Gayle, does the 2nd goal go in? Loses points for trying to be involved in conceding the 3rd, but Palace didn't need him due to other teammates fecking up).