The 24-year-old was shown a straight red card by referee Chris Kavanagh after he was adjudged to have spat at Wigan’s Nick Powell, having reacted to a challenge from the midfielder. Television cameras picked up the moment that led to Masuaku’s dismissal, and Moyes confirmed afterwards that the club will take action against the Frenchman.
"He will deserve everything he gets and he will get something he deserves from us. It's unacceptable," said Moyes.
"It was despicable. I've not seen it from him. We like him, he's a really good boy around the club. I've enjoyed him so I'd have to say it is out of character."
Masuaku is likely to receive a six-match ban from the Football Association unless he challenges the sending-off, while West Ham could fine the player, with the club also able to suspend him if they deem the incident worthy of such a punishment.
Leeds United forward Samu Saiz accepted a six-game ban for spitting in their FA Cup third round defeat by Newport County earlier this season, with Masuaku likely to receive the same sanction.
The distasteful incident marred what was already a bad day for West Ham, as the 2-0 FA Cup defeat was preceded by the loss of Pedro Obiang to injury, with the midfielder suffering possible damage to his knee ligaments that could see him sidelined for the foreseeable future.
“Pedro looks as if he's got a medial knee ligament [injury],” Moyes added.
"We'll need to get over this quick and get on with it, but that's the job. Premier League games come around quickly and we'll get some players back fit, but obviously we've lost a couple more today."
The absence of Obiang takes West Ham’s injury list to 11 players, with Andy Carroll and Manuel Lanzini among that are sidelined with long-term problems that will not help Moyes’ efforts to keep the Hammers in the Premier League.
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