It was the last news Inter needed as they lie nine points off the Serie A pace of arch-rivals AC Milan after 13 games and have a lengthy injury list.
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Serie A said in a statement that Eto'o's headbutt to the chest of defender Cesar, which was not seen by the officials but picked up on television replays, was "premeditated" and constituted "violent conduct".
The incident was similar to the infamous headbutt France's Zinedine Zidane committed on Italy defender Marco Materazzi in the 2006 World Cup final.
Under pressure coach Rafael Benitez said after the match that Eto'o was provoked by Cesar, who hit out at the striker earlier in the match but has escaped punishment.
Inter announced they would appeal against the decision but captain Javier Zanetti was less sympathetic in a news conference.
"Eto'o has enough experience and knows he made a mistake," he said. "He committed a big error and is being punished."
Inter's Cameroonian top scorer was also fined 30,000 euros and if his appeal fails he will not play in Serie A again until next year given the European champions are in action in the Club World Cup in December.
MILITO INJURED
Media reports have said Inter coach Benitez could be sacked if the holders lose Wednesday's Champions League game against Twente Enschede but club president Massimo Moratti has distanced himself from the speculation.
However, even if Inter win on Wednesday when Eto'o is available, their next three domestic games have suddenly become even more complicated without the league's joint topscorer and Inter's only standout performer in a so far lacklustre campaign.
Benitez refused to discuss Eto'o's ban in his Tuesday news conference as he sought to concentrate on the Twente match.
"This could be the first game of a winning run for us, that's how I see tomorrow's game. I am positive," he said.
"Let's wait for the injured players to return and have faith. We can still win lots, our aims have not changed."
Under Jose Mourinho, Inter won the treble of league and cup domestic double plus the Champions League last season but a sixth straight Serie A crown already looks unlikely.
Last season's top scorer Diego Milito is among the raft of first teamers out injured so Goran Pandev looks set to be the only recognised striker for next Sunday's game with Parma.
Precedent in Serie A suggested Eto'o could have faced as much as a five-match ban but he has received just one game more than Juventus winger Milos Krasic, who was suspended for two games last month when television evidence showed he dived.
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