Claude Puel felt Southampton dropped two points after the opening-day battle of the new managers finished 1-1.
Watford, with Italian Walter Mazzarri in charge for the first time, took an early lead through Etienne Capoue but new signing Nathan Redmond equalised in the second half to earn Saints a point.
The Hornets had to play the final 15 minutes with 10 men after Ben Watson was sent off for hauling down Shane Long, and Frenchman Puel felt his managerial debut in England should have ended with a victory.
"I feel disappointment, I think we lost two points,'' he said. "You could see a good second half from my players but the first was no good. But I liked the reaction of the players.
"Watford had one opportunity for the opener and it was too easy for them to score. But in the second half it was one team in defence and one team in attack. Yes, I think we deserved to win after our second-half performance. But it's a good lesson for the future.''
The opening day often throws up the odd surprise and the identity of Watford's goalscorer was certainly one of them. Despite his attacking tendencies Capoue did not find the net once last season, much to the amusement of his teammates, but the former Tottenham midfielder took just nine minutes to get off the mark this term.
Southampton improved vastly after the break with summer signing Redmond increasingly a threat.
The former Norwich winger forced Heurelho Gomes to tip his curling effort inches past the far post and from the resulting corner grabbed a deserved debut goal. Gomes came out to punch the cross clear, only to send it looping high in the air towards Redmond who volleyed it straight back past the keeper and into the net.
Mazzarri, who showed as much energy on the touchline as his players did on the pitch, said: "I'm very happy, especially with how the team played. They played the way I like, with 100 percent energy. I have only been working with them a month so I try to transmit my ideas in this way.
"In their second half we did not have a lot of opportunities. Unfortunately we could have closed the game out in the first half but we didn't.''
Mazzarri was speaking through an interpreter after the game but his response to being quizzed about Watson's red card did not require translating. "No commento'', he added.