Mauricio Pellegrino has vowed to get to the root of Southampton's struggles in front of goal.Saints did not manage a single effort on target until stoppage time as they slipped to a 2-0 defeat at home to Watford.It meant the shot-shy hosts drew a blank for the third time in their four Premier League matches so far this season.Yet the problem dates back back to long before Pellegrino arrived as manager - Saints have now not found the net in nine of their last 12 league outings.Pellegrino said: "It was tough for everybody but we have to keep going and keep working."I am here because the team was in trouble last season. If everything was perfect there would be a different manager here."In the other games we created a lot of chances and could have won the games. Today was the most difficult game."Obviously I'm not happy with our performance. But t o score goals is not just for the strikers." Watford showed Southampton just how to shoot as long-range strikes from Abdoulaye Doucoure and Daryl Janmaat lifted them into the top four. Neither goalkeeper had been forced into action until seven minutes before half-time when Saints only half-cleared a ball into their penalty area. It fell to Doucoure 20 yards out and the French midfielder lashed a right-footed volley past Fraser Forster and into the bottom corner.A Watford defence already hit by injuries was stretched further when Younes Kaboul and Kiko Femenia both limped off on the hour.But rather than knocking the visitors out of their stride, the enforced changes paid immediate dividends as one of the substitutes, Janmaat, expertly found the far corner from 25 yards out.Pellegrino sent on Shane Long and Charlie Austin in desperate search of a goal, b ut the only cheers from the home crowd were the ironic ones which greeted the first shot on target, from Cedric Soares, in time added on. "We deserved three points, we controlled the match which is not easy to do at Southampton," Hornets boss Marco Silva said. "I'm really happy with the attitude and the commitment of the players."It can happen in football. The first goal was a moment when we worked really hard. And the second goal is a great finish, a moment sometimes you might not expect."Meanwhile, Pellegrino revealed Southampton captain Virgil van Dijk, whose request for a transfer was not granted this summer, will play for the reserves on Monday as he continues to build up his fitness ahead of a return to the first team.