The Hornets, who have otherwise made an impressive start to life under Silva in the league, lost the second-round tie 2-3 to the Championship side at Vicarage Road. Hornets also finished the game with ten men as left-back Jose Holebas picked up a red card late in the game.
"Bad night - a really bad result for us, a disappointing result," head coach Silva said. "Of course we prepared for the match to win and we knew what we needed to do to win, but we didn't do it.
"In the first half we controlled the match but we played very slow. Very, very slow, too slow all the first half. We controlled the match, we don't give a lot of solutions for our opponents to make problems for us but we need to do something different in our fast decisions, in our counter attack as well, to create more chances to finish.
"We didn't do that during the first half. We changed some things at half-time, even the positions of our three midfielders.
"We started well, not only because we scored the goal but because we played different, played a little bit faster. And after the goal we came back again for the same style: too slow, everything too slow, we start to lose easy balls, to give our opponent some dangerous counter-attacks with their fast attack."