An addition game was added to the automatic three-game suspension for what an FA disciplinary panel deemed to be a "frivolous" appeal.
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While it would have no effect on the first three games, at Arsenal on Sunday and then the following Premier League matches against Portsmouth and Aston Villa, Manchester United could succeed in overturning the fourth match, at Everton on February 20.
"We have to decide whether to appeal because there are conflicting ways in which these decisions are being viewed," Ferguson said.
"I watched the game between Leyton Orient and Charlton on Monday and there was an elbow on the Charlton player three times as he was running with the ball.
"But nothing has happened with that.
"There is a lot of confusion there, although we accept that for Arsenal, Rio is banned."