Atletico Madrid manager Diego Simeone rued his side's missed chances in Tuesday's match, calling the 3-2 defeat away to Olympiakos "a very negative result for us."
Los Rojiblancos fell two goals behind after half an hour and though Mario Mandzukic cut the lead to one shortly thereafter, Simeone's men could not find the equaliser.
Olympiakos scored on three of their four shots on target and had just seven shots total, while Atletico failed to put a way a host of good chances, despite taking 19 shots and controlloing 60 percent of possession.
"We posed a threat throughout, but Olympiakos were very tough indeed," Simeone said. "What made the difference was that they were more clinical with the chances that came their way.
"There is no doubt this was a very negative result for us and Olympiakos are now in a very good position. We feel we have a responsibility to bounce back."
Midfielder Koke said Atletico made too many mistakes, before setting his sights on his side's next Champions League matchup.
"We made some errors. They were very focused and capitalised on our mistakes," Koke said. "We have to keep working hard and think about our next game, at home to Juventus, who are our biggest rivals in the group.
"We knew that they were going to put us under a lot of pressure with their fans behind them. They scored very early on and it's always difficult when you go 2-0 down.
"We knew that this was a difficult group, and the fans here really get on top of you. The game against Juventus is going to be very important, we have to keep working hard."
Olympiakos coach Michel was delighted with how his players performed against last season's Spanish champions and European finalists but cautioned there are still "long struggles ahead."
"We forced Atlético out of their comfort zone, which is what they usually do to their opponents. We played like a team, like Simeone's side does," Michel said. "All of my players understood what they had to do. We are very happy.
"We tried to spoil their game and we succeeded. Atletico haven't conceded three goals [in a game] in a very long time."