Germany, who beat Portugal in their first Group H game, have a history of struggling in the second game at major tournaments.
They lost to Serbia in their second group match in South Africa four years ago and in 2002, 1998 and 1994 drew after opening victories. They lost as well at the 2008 European Championships.
"From a mental point of view you have to pay a lot of attention," said Loew on the eve of the Group G match in Fortaleza.
"We saw it in 2008 and 2010 when we lost the second match after winning the first. We don't want this kind of problem again.
"Italy has just seen it against Costa Rica and the Netherlands struggled as well against Australia. It's great to win the first match but you have to continue. One victory is not enough, you have to keep your concentration."
Loew said that Ghana, who lost their first game 2-1 against the United States, would be playing for their lives.
"For them it is a final. If they lose it will be nearly over. They are going to fight until the end. They are strong, they are athletic. They attack, they have a lot of players at very good clubs, we know them well.
"Even if it is very hot, they are going to run for 90 minutes," warned Loew.
Victory for Germany would virtually assure them of a second round place. Portugal play the United States in their second match .