Liverpool took a first leg advantage in a dominant display at Anfield. The reds won through 2-0 but could have had more - with David De Gea in excellent form especially in the first half.
Daniel Sturridge scored from the spot in the 20th minute after a foul by Memphis Depay, whilst United buckled under more second half pressure - with Firmino grabbing yet another goal in a purple patch for the Brazilian.
Nathaniel Clyne came back in to replace Flanagan (ineligible for this competition). Sturridge came back into the side - James Milner missing out with a virus - so the reds had an attacking line up.
Liverpool opened to thunderous noise from around the ground. Jordan Henderson picked up a yellow card for a high challenge early on but despite that the reds started brightly. In the 20th minute Lallana was fouled by Depay just inside the box and Sturridge dispatched the penalty past De Gea.
De Gea wouldn't be beaten again in the half as he single-handedly kept United involved - the reds had some solid chances. Coutinho perhaps could have made a chance at the back post more difficult, while their Spanish keeper did well to pull off a reaction stop to a Lallana chance.
In the second half the reds were kicking towards the Kop and it was the reds who started better. In the 50th minute United tested Mignolet for the first time - their shot heading straight towards our Belgian keeper. Just around the hour mark United enjoyed a brief spell of possession but the reds eventually broke clear.
Sturridge was replaced shortly after by Joe Allen - as the reds looked to beef up their midfield numbers and the tactical change did improve the reds.
Lallana was involved in the reds second which was coolly slotted by Roberto Firmino to give the reds a more respectable scoreline.
Marouanne Fellaini dodged a second yellow card all evening - an elbow on Emre Can was missed by the officials but could be picked up by UEFA.
Liverpool take a two goal advantage to Old Trafford where the operation must be to get an away goal and leave United needing four to go through.