The 27-year-old winger believes Sir Alex Ferguson's side still can't take anything for granted despite their significant lead in the table, given last year's late dramatics
Roberto Mancini's side closed the gap at the top of the table - with seven games to go - back to 12 points after emerging 2-1 victors from Monday's derby clash at Old Trafford, courtesy of goals from James Milner and Sergio Aguero.
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"We're a little worried. It has already happened to us," Valencia told Ecuadorian radio station Sonorama.
"Last year, having eight points with seven games to go we lost the league, and now we have seven games, 12 points and we don't want this to happen again.
"We want to finish off the two games that are coming as soon as possible, starting off in the right way and ending up victorious."
The Ecuador international admitted he was disappointed only to feature for 10 minutes as a late substitute during the defeat to City and that he is growing frustrated with his lack of action recently.
He added: "They are the manager's decisions [to make], but I was still a bit upset.
"In such an important game, to be not taken into account as a starter is upsetting.
"Ten minutes is very short for such a tight, difficult game. The important thing is to come on and play, but yeah, you become restless waiting for the next game and wanting to play 90 minutes."