“You cannot make any mistakes, there’s no relaxing, otherwise you’ll pay for it”
“So we’ll have to be very careful and stay very focused to not allow that to happen.”
City and Barcelona are favourites to cruise through Group C ahead of Celtic and Borussia Monchengladbach, who Pep Guardiola’s side beat 4-0 in Germany.
But Fernandinho added: “The Champions League is always special, you always have to be careful.
“You always have to think that you cannot make any mistakes, there’s no relaxing, otherwise you’ll pay for it.
“That’s our intention this year, to play 100% concentrated, the way we have been playing, and try to win these games which are really important to us.”