Premier League side West Ham trailed to an early Will Grigg goal when Masuaku was dismissed four minutes into the second half.
Replays indicated Masuaku appeared to spit at Powell and Moyes confirmed after the match the player had acknowledged his indiscipline.
"I asked him did he spit? And he said yes," Moyes told reporters.
"Arthur - what he did was despicable. He will get something [a punishment] 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 [to send Masuaku off].
"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."
Grigg converted a second-half penalty to secure a place in the fifth round for Wigan, while West Ham's misery was compounded by an injury suffered by Pedro Obiang.
"We think it is medial ligament," Moyes added. "But we will have him sent for a scan to tell us exactly what’s wrong."