The Ospreys would guarantee their top-four spot with a bonus point win over the Dragons, who have been in poor form recently, following back-to-back home defeats to Connacht and Treviso.
But former Wales captain Humphreys believes the Dragons will be a force to be reckoned with, calling on the Ospreys to ensure Williams' swansong is a successful one.
"Hopefully we can give him the send off he deserves," added Humphreys.
"He's been one of the most influential players in Welsh rugby history.
"The Dragons play pretty well against us. They've got some great players and it always baffles me that they're in the position they're in. If we don't play to our best then we'll come unstuck.
"The Dragons will be a little sterner opposition than we faced last week."
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