Rehhagel's side were a shadow of their Euro 2004 winners
Greece coach Otto Rehhagel blamed a lack of passion from his side for their 2-0 defeat by Sweden at Euro 2008.
Greece struggled to mount any kind of goal threat as they started the defence of their Euro 2004 title with an unconvincing showing in Group D.
"The Greeks didn't play with heart like they normally do, and the Swedes pushed us hard," said Rehhagel.
"Some players were unable to play at the level I expected. They tried hard but it wasn't enough," said the German.
Greece now face Russia in their next game on 14 June before facing Spain four days later and Rehhagel knows his team will need to improve to reach the quarter-finals.
"The first half didn't go as we planned - we didn't play high enough up the pitch and then we lost Zlatan Ibrahimovic for a moment, which left us a goal down.
"We will have to show a tougher side of us in the next match."
Greece made a fine art of disciplined defending and catching teams out on the counter-attack or at set-pieces on their way to the Euro 2004 crown.
But their defensive tactics did not pay off against Sweden and Rehaggel said: "I did what I thought was right. But if we play like that we won't win the tournament again.
"I didn't want my defenders to keep the ball at the back for an extended period of time, I didn't want them to do that.
"We still had the opportunity to win after half-time but we have to admit Sweden won the tackles and had the upper hand.
"They simply played too quickly and our midfield play took too long."
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