The most important thing for Germany is not to make mistakes against Costa Rica later.
It is a must if they want to have a chance to qualify for the next round. If they can beat Costa Rica dan Japan is defeated by Spain, Germany will be in 2nd place by getting 4 points. But if they get a draw result or are defeated by Costa Rica, Germany will automatically is eliminated in the group stage.
There were several impressions in my mind during the first match as if they underestimated Japan in the match they did which actually backfired on them. when this happened again in the game against costa rica then i'm not too sure if they can win or not.
I am not sure if they underestimated Japan, I assume Germany lose focus in the first match. In the world cup tournament, the first match often gives surprising results because some teams may feel nervous or lose focus because of many factors. In this world cup tournament, not only Germany, even Argentina was also defeated in the first match. But they can get a positive result in the second match. Sadly, Germany can't do it since they must play against a strong team like Spain.