Japan, seeking a record-extending fifth title, have conceded in all four games at the tournament, with an own goal when they were 2-0 up giving Bahrain a glimmer of hope in the last-16 tie before the Samurai Blue wrapped up a 3-1 win.
"Obviously, we wanted to keep a clean sheet today. But it is what it is. We need to try and keep a clean sheet in the next game," Tomiyasu told reporters.
"As a defender, I want to keep a clean sheet every time on the pitch but today we couldn't manage it. I'm not happy about this."
Although Japan coach Hajime Moriyasu said he was pleased with his players' performances, he added that they were not playing at 100% and Tomiyasu agreed.
"I think we're not at our best form now. We could have done better today as well, we should have done better - especially in the second half," Tomiyasu added.
"We needed to kill the game at 2-0, but we gave them hope when they scored. We need to manage the game well and close the game out."
Tomiyasu plays as a full back for Premier League side Arsenal but he has been deployed in central defence by Moriyasu after returning from an injury.
"Obviously, I got injured and I missed the first game, but I'm fit now. My condition is getting better," the defender said.
