Wayne Rooney admitted Manchester United have lost their fear factor at Old Trafford after they slumped to a 3-0 derby defeat against Manchester City last Tuesday.
United's recent revival was ended in crushing fashion, with Edin Dzeko opening the scoring after just 42 seconds in front of the stunned home fans.
Dzeko added his second after the break and Yaya Toure's late strike condemned United to their sixth Premier League defeat at Old Trafford -- more than in the previous three campaigns.
Asked why United had struggled on their own patch, Rooney said: "I don't know but it's not good enough.
"We can't lose six home games in a season and we have to put that right and make this a place teams fear again.
"We know we have to respond. It's a bad night for us all but we have to move on."
Juan Mata wasted United's best opening in the first half, while City goalkeeper Joe Hart denied Danny Welbeck from close range after the break.
Rooney felt the hosts lacked a cutting edge and cursed their sluggish start to the match.
