Kubo, after starring in victory over Germany, didn't featured against Costa Rica. The result means Japan need a draw against Spain to reach the knockout round.
"Perhaps too many (changes), perhaps undervaluing Costa Rica more than necessary," Kubo admitted to Mundo Deportivo.
"They played us a reverse match. Just like us against Germany, they did the same to us.
"Now we all have to play Spain. It's the most important game of my life. When I'm fresh, I'm sure I'll be brought back and see if I can score the winning goal.
"The best thing about Japan is that we are capable of adapting to any rival. We can play against the greats. We are very competitive. If there is a match against Spain, Brazil, against whomever, I don't think we will lose by more than three goals, because the team does not disconnect."