Namo originally received a grade F charge from the match review panel for dangerous contact following an incident that left Wigan Warriors back-rower Willie Isa nursing a serious ankle injury.
The charge was downgraded by the independent operational rules tribunal but the prop has been hit with a lengthy ban, as well as a £750 fine.
The club are weighing up whether to appeal after seeing Namo’s not guilty plea rejected.
“Sylvester Namo has had his charge downgraded from Grade F to Grade E and has been suspended for five games and has been given a £750 fine,” read a statement from Castleford.
“The club will be reviewing options in detail tonight and will provide an update tomorrow.”
Jesse Sue’s one-game ban stands after Hull KR failed to successfully challenge the grading of his dangerous contact charge but Catalans Dragons forward Bayley Sironen has been cleared of grade E head contact.