La Roja are considered major contenders to win gold, but the former Real Madrid man is taking nothing for granted in light of the recent shock 2-0 defeat by Senegal
La Roja are considered one of the favourites for the gold and they are going into the game in Glasgow on the back of a morale-boosting 1-0 win over Mexico on Wednesday night.
However, they were defeated 2-0 by Senegal in their previous warm-up clash and Milla feels that particular result is of greater relevance.
"Japan will be similar in style to Senegal: very dynamic, very physical, but with forwards and midfielders who are playing at a high level in Europe," the former Real Madrid man told reporters on Friday. "They are going to be a team that is very well prepared.
"So, I'm thinking only day to day; the game against Japan is the most important game. If you have a poor 10 or 15 minutes, you're going home."
Spain also have Honduras and Morocco for company in Group D.