"For sure now it is our worst performance in the Premier League [this season]," said Marco Silva after the game.
"We didn't perform really like we need to. We started the game too slow.
“For the second and third goals we made mistakes at this level we cannot make. It was a tough afternoon for us."
Watford's ineptitude could not be excused but they were unfortunate for the first goal as both Kachunga and his provider Collin Quaner looked offside.
And Watford captain Troy Deeney was also unhappy with his sending off as the striker believed he touched the ball.
"Of course I spoke to Troy, he has spoken with the officials as well," Silva added.
"He said he touched the ball, that's what he told me. I think the referee answered him 'it's not because you take the ball, it's because of the speed'."