Manchester United manager Louis van Gaal said his side had given away "cheap" goals in their dismal 2-0 Europa League defeat at Liverpool.
United face an uphill challenge to get back into the round-of-16 tie after a Daniel Sturridge penalty and Roberto Firmino rewarded Liverpool for their dominance at Anfield.
Van Gaal said his men were now in a "difficult" position ahead of the second leg at Old Trafford next Thursday.
United fell behind when Memphis Depay was adjudged to have pulled back Nathaniel Clyne for the penalty -- a decision Van Gaal disagreed with.
The United manager also said the assist for Liverpool's second goal, scored by Roberto Firmino, had been offside.
He told BT Sport: "I have to say Liverpool created an atmosphere that was fantastic. In the first half, we couldn't cope with Liverpool's pressure. David De Gea was fantastic.
"But it was a cheap penalty. He [Clyne] was held outside the box then he fell down. But it's difficult -- you can't see it in one second. How their goals came was cheap.
"In the second half, we changed the shape then made much more contribution to the game, but we didn't create many chances. They then scored another cheap goal.
"Two-nil is a very difficult score for us. We have to create an atmosphere like Liverpool have done."
Talking about the second goal, Van Gaal said: "The guy who plays the pass for the second goal [Adam Lallana] was already offside. When you give a sharp penalty, you then have to be sharp with the offside."
United's problems could mount in the aftermath of the match, with Marouane Fellani, unseen by the referee, appearing to have elbowed Emre Can.
Van Gaal said: "I have not seen that, but Fellaini was one of the best players on the pitch. I'm very sorry when he makes a mistake like that."
United's performance was criticised by former players Rio Ferdinand and Paul Scholes as being "disjointed" and "shambolic."
However, a tetchy Van Gaal rejected their opinions.
"Is it important what Rio Ferdinand is saying, is that important to you? You don't give your opinion and then you give Rio Ferdinand's opinion -- that is very strong of you,'' he said.
"Of course Liverpool were the better team in the first half but in second half we did a better match than before that.
"I don't think we were very creative in the third and fourth phase but it is also the defence of Liverpool and you can give credit for that to Liverpool.
"We have to improve a lot to create more goals."