The former sevens star was sent off in the 25th minute in Cardiff for a dangerous tackle on Johnny Williams.
The Flying Fijians were leading 13-7 at the time but also conceded two yellow cards and eventually lost the match 38-23.
Sau appeared before an independent judicial committee via video link and accepted he made direct contact to the head of his opponent but denied that he had commited an act of foul play, arguing his red card was not justified.
The committee ruled that Sau made direct contact to the head of his opponent, with force and with a high degree of danger.
The mid-range entry point for foul play resulting in contact is a six-week suspension but the committee reduced the sanction to five weeks, after taking into account Sau's apology to his opponent and for his conduct during the hearing.
The 31-year-old will miss the Flying Fijians' final November test against Georgia this weekend as well as four matches for his club side Provence in the French second tier.
However, he can reduce his suspension by one game if he successfully completes a head contact process coaching intervention.
Eroni Sau has 48 hours to appeal the decision.