Before scoring twice in the visitors' 4-2 defeat at Liverpool, Semenyo reported to ref Anthony Taylor on 29 minutes that he had been racially abused by a Liverpool fan.
Taylor halted the game to speak to both managers and also captains Smith and Virgil van Dijk before the match was resumed.
Afterwards, Smith told Sky Sports: "Totally unacceptable. Kind of in shock it happened in this day and age. I don't know how Ant has carried on playing and come up with these goals.
"He's a little bit down - something needs to be done. Taking a knee has taken no effect. We've supported him and hopefully he'll be OK.
"I wanted him to react (after scoring) - that's what I'd have done. I'd have gone straight over there. It shows what kind of man he is to report it to the ref and carry on. Fair play to Ant."
Smith also said: "I said to the ref I wanted him removed immediately but the police went and sorted it. The Liverpool players were very supportive to Ant and the rest of the team. Just so angry.
"We've had discussions with the Premier League about it so they have been taking it seriously. I don't know what else we can do.
"We've been doing it for a long time now and no-one is getting it. I just feel sorry for Ant and he's had to take that. The whole country is watching and it's shocking."