"It is clearly one of my most disappointing days as manager of the club," Benitez said.
"It is a game I know everybody at the club and the supporters wants us to win, particularly at Old Trafford. I felt we started the game well but we didn't create enough. Things were okay until we conceded the first goal.
"That changed the game for both sides. We needed to go forward, the game was more open and with their quality it became more difficult for us," he added.
Benitez concluded, "We must focus on improving, getting better at what we do - and the things we are not doing well."
