The Portuguese felt that his side took their foot off the gas against Hull City Tigers on Sunday, while he believes Manchester City are "big contenders" to win the Premier League
The Europea League winners won their opening Premier League fixture against Hull City Tigers but failed to add to the scoreline after registering an early two-goal lead.
Manchester City, meanwhile, struck four times against Newcastle to leap to the top of the table and Mourinho has urged his side to match that ruthlessness when they take on Aston Villa on Wednesday night.
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"If someone has the chance to do it to you, they will not forgive you. They will give you nothing. If someone feels that they can do it to us, they will do it. So, if we have the chance to do it, we have to do it.
"If we do that, we destroy an opponent, we get a much bigger result because going that way you have to score goals, even if the opposing goalkeeper saves a penalty."
Mourinho watched Manuel Pellegrini make his bow in English football on Monday evening and, impressed by what he saw, believes that City can mount a serious title challenge this term.
"I watched the game and they were very good. No doubt about it. They bought fantastic players, they have a fantastic squad and they looked very, very good," the Portuguese observed.
"When you look at the players they didn't have on the pitch - and some were not even on the bench - the squad is amazing. They are a fantastic team and an amazing squad so, yes, they are big contenders but, for me, I think we start, all six, on zero points."