A few things.
1. We need to STOP building the attack around Benteke. It's difficult when you've spent 32.5 million on him, but he's not our best attacking asset. Our best attacking players are our floaty AMs - Coutinho, Firmino, Lallana and Teixeira (not Ibe yet). Build around them i.e. get them playing in central positions, not out wide. The only way this happens is through a diamond - there just isn't another formation that accommodates them at present.
This means getting Benteke off the ball, and making Coutinho and Firmino the centre of our atttack.
Basically this:
--------------Coutinho
-------Firmino--------Benteke
This brings the best out of Coutinho, and Firmino gets ample opportunity on the ball. Playing him out wide is a major major mistake. This also lets you bring in Sturridge or Ings for Benteke seamlessly. It also lets you try Benteke and Sturridge together if you want to.
2. The 4-4-2 diamond is a narrow formation, and that's going to be a problem against teams who clog the centre like West Ham did against us.
That said, what we played against West Ham wasn't a 4-4-2 diamond but a 4-3-3 with Benteke as a pivot. That's a terrible idea - because the way you beat a team which clogs the centre is by using your full-backs to stretch the pitch, and filling your own centre with quick, technical, slithery players. What I would've done against West Ham was - taken off Benteke completely. Brought on Ings and pushed him up top with Firmino. Put Coutinho behind them. Moreno instead of Gomez to stretch the pitch down the left
The likes of Winston Reid will defend against Benteke all day long. Benteke can't turn and slip past them - he doesn't have that trickery or quickness. Ings/Sturridge and Firmino would've been a different proposition altogether, especially if we had the ability to stretch the pitch down both flanks.
If I wanted to be even more attacking, I would've taken off Lucas for Ibe and gone with a proper 4-4-2. The basic point being to fill the side with smaller, quicker and more technical players.
3. Use Benteke against teams which have soft CB pairings. He's ideal for the Mancs who'll play Smalling and Blind at CBs - easy to bully.
Use the diamond against them - I don't mind being narrow against the Mancs - their full-backs are quite decent, so trying to stretch the play against them isn't going to yield much dividend. Go with this:
--------------Firmino
-------Ings--------Benteke
Two forwards to keep their CBs occupied, and our most creative player behind them through the centre.
I'd also drop Lucas - we need to go for the jugular against the Mancs - they aren't very good, and are as brittle mentally as us. This game will go to the side which backs itself and is more aggressive.
So this'll be my side:
---------------------------Mignolet
Clyne---------Skrtel---------------Sakho-------Gomez
-----------------------------Can
--------------Henderson---------Milner
--------------------------Firmino
------------------Ings-------------Benteke
4. Get Teixeira into the squad asap. With Lallana injured, and Markovic out on loan, we need someone to deputise for Coutinho. Teixeira is more ready for first team action than Ibe IMO.