Arsenal defender Laurent Koscielny says Shkodran Mustafi was very much needed ahead of upcoming fixtures in the Premier League and the Champions League.
The Germany international is expected to be officially announced at the start of next week and will add depth to Arsenal's defensive department.
The Frenchman believes a "big squad" is needed if the Gunners are to be competitive in the domestic league and in Europe.
"I think it was very important [to win]," Koscielny said after the game.
"We didn't start the season well but we showed some good things at Leicester and today we wanted to finish the job before the international break to get the three points.
"We played very well between the lines, they played with five defenders and we were better. In the second half, we were a bit asleep and needed to wake up. We gave the ball to Watford and they can play with it, but the most important thing was to get the three points.
"[New signings?] First of all, we have a few people injured.
"When you start the season with Welbeck injured, Mertesacker and then Gabriel out, it is difficult. You need to have some players, because when you play every three days in September, October and November you need a big squad to fight in every competition.
"I am sure the boss will buy some players to help us."