Liverpool are 15 points adrift of league leaders United and are out of the Champions League.
“It would be very dangerous to think this is a poor Liverpool side coming to Old Trafford because they are anything but,” Fletcher told the Manchester Evening News.
“It would be a huge mistake to think that Liverpool are not a threat. They maybe haven’t had the season we expected after last year but they are still a dangerous side.
“They are still in the main the same team. We know what they are capable of and pushed us all the way and dished out that defeat here and the one at Anfield this season. They are still fresh in our memories.
“Liverpool still have a lot to play for because they are battling for fourth spot. They are capable of beating anyone.
“We have to learn the lessons from last year’s game. The warnings are there. If you are not 100% focused and you make mistakes they will punish you.
“It is a real surprise to me they haven’t been up with us. But they will still be fired up to win at Old Trafford again.”