City won 2-0 at Old Trafford courtesy of second-half goals from Bernardo Silva and Leroy Sané, which consigned United to a seventh defeat in nine games.
The result leaves City a point clear at the summit with three games remaining, making Pep Guardiola’s side confident of retaining their crown.
Yet for United, the third consecutive home defeat to City once again exposed the gulf in class between the two sides.
Ole Gunnar Solskjaer’s men were criticised for their performance in defeat to Everton on Sunday and Rashford felt their showing on Wednesday night was not much better.
“Today we didn’t play like Manchester United. It’s not right,” he said. “The bare minimum is to work hard and give all for the badge, the supporters, the club and yourself.
“It has been a tough road to get to where we are, to get there and not be putting up the same performances as we were in January is disappointing.
“We know what’s been going wrong, it’s mentality and willingness to run for your team.”
Despite yet another defeat, United remain in the hunt for a place in next season’s Champions League after Arsenal lost away to Wolverhampton Wanderers.
United are three points behind fourth-placed Chelsea, who drew against Burnley on Monday night and visit Old Trafford this coming Sunday.
Rashford, however, believes United need to put all targets and aspirations to one side and concentrate on returning to form.
“Forget the league and top four, you don’t lose this game. It’s that simple as a Manchester United fan,” he said.
“We never call each other out but we have to be real with each other. It’s a matter of trying to improve each other.
“It’s more getting back used to doing it on the pitch. They [the supporters] know we can do it.
“Forget the top four and the Champions League. We have to get to the bottom of it and start playing like Manchester United for ourselves.”