United were patient before ramming home the advantage after the interval as Will Keane, birthday boy Jesse Lingard and Mame Biram Diouf (2) all got on the scoresheet.
Borrell told liverpoolfc.tv: "I thought we started the game quite well but the red card made it a very difficult night for us against a very experienced side.
"I need to see the incident with Suso before commenting on it because I haven't seen it yet. To play with 10 men against a side like that is not easy. The red card changed the game."