Both sides went into the clash in the top six, but it was the Tigers who looked a class above against a below-par Hornets, whose seven-match unbeaten run came to an end.
Former Italy international Cassetti, 35, applauded the Tigers' quality and hopes Watford can respond to the defeat in the right manner.
"We lost against a very good team," he told the club's official website.
"This is the truth. We cannot cry because we didn't play very well and, for me, Hull City are the best team we have played against this year.
"They were very organised, had good players and were solid. All afternoon, it was difficult for us to play against them.
"The second half we had some chances. (Almen) Abdi hit a post and then Nat (Chalobah) had one saved on the line. Sometimes we will need some luck to be able to draw this type of match, but maybe it's not good for us to get anything because we didn't play well so we must learn from that."
Cassetti added: "We need to learn the lessons ready to start the the next game. Remember this game for the future. If we are in the same situation, we will deal better with the challenge."