Marco Silva has claimed that Olympiacos were punished for not taking any of their chances in the 3-0 loss to Bayern Munich on Wednesday night.
Bayern started the Group F campaign with a victory in Greece as they held off the hosts with two late goals to put the result beyond doubt.
Brown Ideye and Leandro Salino both missed good chances either side of Thomas Muller's opener, and Silva believes that the wasted opportunities were key.
"It was a disappointing end for us and it's left a bitter taste because we played well," the Olympiacos boss told reporters. "We were unlucky with the first goal we conceded, and if we had taken our chances things would've been different.
"Against such difficult opponents it's always hard to prepare. The players were given new roles but we are pleased with how they did. If I have one criticism, it's that we were lacking pace and our crosses weren't good enough. We didn't take our chances and we paid the price."
Olympiacos will visit Arsenal in their next Group F match on September 29.