The Greeks conceded twice inside the opening six minutes against the Czech Republic in Tuesday's 2-1 defeat.
The same slow start afflicted them in their opening game draw with Poland where they went behind in the 17th minute.
Against the Poles, Greece fought their way back into the game but they left themselves too much to do against the lively Czechs and defender Jose Holebas said they can ill-afford another sleepy start.
"Every single one of us needs to have their mind on the game, and right from the start," he said.
"We must have the will to win the game and not only in the second half. We played better then and created some chances but this must happen from the first minute. You have to take control of the game to win it," he added.