Lionel Messi (three) and Neymar scored in a 4-0 win for the hosts, while City also had Claudio Bravo sent off for a handball outside the box.
Nolito said: "We played well in the first half, we had our chances but in the second half we were unlucky. This is football, we need to keep going. Today was a very hard game against Barca, but we did not deserve to lose 4-0.
"Messi is the best player in the world so he makes the difference. When he gets the ball he always does something, a goal or an assist.
"We go back home sadly, but we must keep going, this is football. We knew that we play against a very tough rival and these things happen in football.
"We must be closer now. This is football. We want to grow step by step. On Sunday we have a very important game in the Premier League and we need to win it.
"In the Champions League we still have three important games and we have to win them."