Play our strongest team as this is arguably more important than the Utd match.
If we lose over two legs against Southampton then that's a cup gone. Lose against Utd and we can recover over the remaining games.
So play our strongest team and worry about Utd afterwards.
We will play our strongest available team for both games. For this one we will be missing Matip, Hendo and Milner (I think Milner is out for this one) to injury, Coutinho will have a place on the bench as he's not quite ready yet so we do have some limited options. It sounds like Karius will start in goal for this one.
All things considered, here's the lineup that I think Klopp puts out.
Clyne Klavan Lovren Moreno
Lallana Origi Firmino
Bench: Migs, Sturridge, Trent AA, Ejaria, Woodburn, Coutinho