Ernesto Valverde said Frenkie de Jong's sending-off hurt Barcelona at a crucial moment in its 2-2 derby draw with Espanyol and noted his side's difficulties in killing off games.
The Blaugrana came from a goal down at RCDE Stadium to lead 2-1 thanks to goals from Luis Suarez and Arturo Vidal but De Jong was shown a second yellow card 15 minutes from time.
It was the midfielder's first career red card in league football and preceded an Espanyol fightback in which Wu Lei scored a deserved equaliser to earn La Liga's bottom club a share of the points.
Valverde, whose side has won just one of its past four top-flight matches, called for his side to battle for victories.
"We have to fight in every match," Valverde told reporters.
"We started badly, we managed to come back but unfortunately we went down to 10. They pressed and equalised.
"Frenkie's dismissal did us a lot of damage. We were unable to score a third. This is football.
"It was a very competitive game, a derby with a lot of tension and emotion. If you're winning and the opponent equalises, you leave with the impression of having lost two points."
Barca has made a habit of coming from behind in recent away games, doing so in its 2-1 victory at Leganes and its 2-2 draw at Real Sociedad.
Valverde acknowledged his players are capable of digging their way out of difficult situations but thinks they have struggled to go for the jugular when it matters.
"In a lot of games, we cash in first," he said.
"We managed to take the lead in four away games, whereas in other games we managed to come from behind.
"We have a reaction capacity, but we are not always able to kill the match. The sending-off affected the match."