Spurs were brought back down to earth after Monday's win over Manchester United at Old Trafford. Abdoulaye Doucoure's own goal gave the visitors the lead before two goals from set-pieces kept Watford's winning streak intact.
And speaking after the game, Pochettino said Tottenham would not challenge for the Premier League title this season if they continued to defend in such a way.
"From two set-pieces we conceded two goals," he told Sky Sports. "If you want to be a contender and to be on top you cannot concede from this type of action.
"I think it is important to clarify the point, first half we were so relaxed, we dominated the game, but we were not aggressive enough. A lot of unforced mistakes. You need to show more strength, be more aggressive in your defending. You need to show you want to win.
"Of course, in every single Premier League game this situation will happen, with any team, and you need to show more strength defending, because that situation is when you really show that you want to win.
"Of course, I'm disappointed with the result, but the reality is clear, this is why we lost.
"I told everyone after the [Manchester United] game that I wasn't happy, and again we try to defend long balls and you need to show that strength, personality, character to play football and defend in the moment.
"I think it is clear from the analysis. It is clear and easy to explain."