The Azzurri lost 26-23 to Japan at the end of June which made in nine successive defeats.
That barren spell includes a disappointing RBS 6 Nations campaign earlier this year where Italy lost all five games and finished bottom of the pile.
Male Sau's bulldozing try on the hour mark proved the difference for Japan against the Azzurri in Tokyo last time out.
And Brunel says there are lessons to be learned if Italy are to reverse the losing trend.
"We made way too many mistakes," he said. "We gave away too many penalties and committed 12 turnovers - you can't beat a team like Japan if you play like that."
Italian captain Quintin Geldenhuys paid tribute to Japan, who recorded their first win in six meetings with his country.
He said: "All credit to them. They are a completely different team from the one we played three years ago.
"They've improved a lot in all phases. Now they're playing in the men's league."
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