The Saints began in a nervy fashion at the London Stadium but soon kicked into gear to come away with a 3-0 victory.
"The first half was difficult," said the Frenchman.
"We looked nervous at the start in a large stadium. It was important to correct that.
"In the second half we could see a team and players with a good quality level and we created many chances.
"It was important for the team and for me that Charlie scored with his first chance. This is a striker with confidence."