Olympiacos head coach Takis Lemonis rued untimely errors after his team failed to capitalise on a red card for Barcelona defender Gerard Pique at Camp Nou.
Barcelona cruised to a 3-1 victory in Champions League Group D, despite Pique collecting a second yellow card for deliberately handling the ball into the net before half-time.
At that stage, Barca were 1-0 up thanks to a Dimitris Nikolaou own goal and the defender hacked down Lionel Messi with an hour played, clearing the way for Barca's talisman to convert a free-kick and bring up 100 European goals.
Lucas Digne made it three before Nikolaou ended an eventful outing by heading in a consolation.
"Barcelona have amazing players and are so compact. However, as the game unfolded, I believe we could have got something more from the second half, especially after the red card," Lemonis said, as quoted by UEFA.com after overseeing a third defeat of the competition.
"That is what I told my players at half-time – remain focused, don't open up.
"However, we conceded a second goal after a needless foul.
"But I think that the team are making steps forward and our tactics are improving."
Midfielder Thanasis Androutsos is hopeful Wednesday's lacklustre showing can be chalked up as a learning experience.
"It was a dream come true for me to play here at the Camp Nou, an unbelievable experience," he said.
"We cannot be satisfied with the result, losing is always bad, but matches like these give us the opportunity to learn from our errors, to work harder."