Masuaku was unhappy with a tackle from Nick Powell 49 minutes into the game and spat in the direction of the midfielder, earning a red card from referee Chris Kavanagh in the process.
Moyes did not hold back when asked about the incident at full time, telling reporters: "It's despicable what he did. He deserved it. He will get something off us.
"Let me not hide behind it. The referee didn't see it, he was influenced by the players but it was the right decision. We gave him a chance, we like him, he's a good boy around the club and I've enjoyed him it is out of character. I asked him did he spit? And he said yes."
Masuaku, who has impressed since Moyes arrived at the London Stadium in November, faces a possible six-match ban when the Football Association reviews the incident.