Mauricio Pochettino was left baffled by Tottenham's drop in intensity against Olympiakos and claimed his players didn't "respect the plan."
In last season's competition, Pochettino's side recovered from taking one point from their first three games to finish second in their group and this early slip up could make qualification tricky once again with Bayern Munich and Crvena Zvezda to come.
When asked what went wrong from Tottenham, Pochettino said: "I think it's easy to explain. When we are not showing that intensity that the competition demands we struggle. We are not talking about quality. We're talking about being focused, concentrated, aggressive, anticipating the action.
"How many times did we anticipate today? How many times we were not proactive like we were against Palace. That is a very good example in three days, it's a massive difference. It's not about tactics, it's not about quality players, it's about the quality of preparing yourself to be ready to fight.
"At this type of level you need to match the opponent in terms of intensity, aggression, in excitement and motivation and then is going to appear your quality when you are connected with the game.
"That's the first demand you need to work with. It's not only the responsibility of one person. It's everyone's responsibility and it's not possible to, in three days, how we start the game today be a massive difference. That is my disappointment. I feel disappointed for everyone."
Pochettino went on to suggest that his players also failed to carry out his instructions.
"From the beginning we had a plan, we didn't respect the plan," he said.
"I told the players at half-time. We scored two goals but if you only analyse the performance it wasn't great. We conceded a lot of chances to them and the first goal was so painful. We didn't translate that type of aggression these type of games demand."