The 28-year-old centre, who helped his side win the Heineken Cup in 2008, received a yellow card for a separate incident in the 13-9 defeat to Leinster and was cited under both law 10.4 (e) for an alleged dangerous tackle and law 10.4 (a) for allegedly punching or striking in the same incident with D'Arcy.
The suspension means Mafi will miss the Pool 3 clash with London Irish at the Madejski Stadium on Saturday when the top two teams in the Aviva Premiership and the magners League go head-to-head in Europe for the first time. He will also be unavailable for the Round 2 match against Toulon.
The Irish Rugby Football Union's disciplinary panel of chairman Hugh Logan, Stephen Hilditch and Ian McIlrath found Mafi guilty under law 10.4 (e) for a dangerous tackle and decided that it was at the top end of entry for such an offence.
The panel took into account the player's previous good disciplinary record before they banned him up to and including Sunday, 21 November. As well as missing the first two rounds of the Heineken Cup, Mafi will also miss three Magners League matches.
The player has the right to appeal against his ban.