"We are very disappointed and to be honest I didn't see that coming as our momentum and confidence coming into the game was probably as high as it's been," said the Reds boss, whose side dominated before Benteke's 29th-minute opener.
"We probably got punished for our decision-making in that first 20 minutes, we were a bit careless. We could easily have been 3-0 up in the game and if we do that we can control it much better but we didn't have that final pass and bit of quality at the top end of the field.
"Then we went on to concede some poor goals. We weren't quite at it with our intensity and pressure in the game and they were a threat on the counter-attack and got the goals.
"I thought we were flat. It was important for us to get the first goal - with them playing 3-5-2 - to open up the game but we had those chances early on and didn't take them.
"Once that happens, combined with sloppy play and giving the ball away in poor areas, we contrived to concede bad goals.
"Benteke was outstanding but he got his shot in too easy for the first.
"The second goal we never tracked the runners into the box and then the third goal we've given it away in our half and he's broken through and nearly runs to the edge of the six-yard box before finishing.
"It was so unlike us because we're usually strong defensively. It was very much self-inflicted. It was surprising because we haven't shown that in the last few months."