Trailing 2-0 from the first leg and with manager Manuel Pellegrini serving a touchline ban, Cousillas was the man calling the shots from the sideline with Manchester City F.C. chasing an unlikely victory.
The Premier League club created chances, but they lacked a killer instinct in front of goal as strikes from Lionel Messi and Dani Alves gave the Spanish side a 4-1 aggregate victory.
"We played a good game against a really good rival," Argentine Cousillas told reporters. "We had chances to get the first goal at nil-nil.
"We usually play with two but against FC Barcelona, with such good midfield players, we thought we needed to reinforce in midfield. But I don't think it would have changed the result.
"In the second half we had three or four clear chances with just one striker, so you can't say if we'd played two strikers we'd have had the same chances.
"Sometimes you score and sometimes you don't. We tried, we had our chances and sometimes you can't convert them."
Manchester City F.C. striker Sergio Aguero limped off injured at halftime and defender Pablo Zabaleta was sent off, but Cousillas was pleased with the team's display.
"Of course, it was difficult with 10 men against such a powerful team but we'll go home happy with the performance of the team," Cousillas added.
"We have to keep working but we're happy with our squad and happy with our performance.
"Players are human beings, not machines, and you have to go through some low times when you play so many games."
Reaching the last 16 of the Champions League was the furthest Manchester City F.C. have ever gone in the competition and captain Vincent Kompany believes they will be better next season.
"Only the winners get remembered but it's only our third year ever for Manchester City F.C. in the Champions League," the Belgium defender told Sky Sports.
"The way we have played means we should fear nobody next season and our time will come.
"It is a matter of time, every team gets its moment."'