The Samurai Blue were beaten 4-1 by Colombia on Tuesday to send them home from Brazil with only a single point to their names.
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"I know many fans were rooting for us and I felt their strong expectations," he told reporters after the defeat.
"But I just wasn't good enough to achieve anything and I feel sorry for letting them down. It is frustrating that the World Cup is now over.
"My career goes on," he replied when asked about what came next for him, with talk of a departure from Old Trafford continuing after two difficult seasons at United.
"It might take some time to accept this but I’ll prepare myself to move on."