Arsenal recovered from their humiliation at Anfield with a 4-1 victory over Fulham on New Year's Day, but Emery is under no illusion about the condition of his squad in the defensive areas.
Despite the impressive scoreline, Arsenal were again caught out at the back and could easily have been two goals behind were it not for errant finishing from Ryan Sessegnon, and Emery confessed he needs to get more out of his defence.
"Our challenge now is to improve defensively," Emery said. "We are very happy with the attacking players and scoring a lot of goals like today. We are high team in terms of scoring.
"We need defence also to take more balance and improve. We need to work. We need to prepare for the second half of the season.
"We need to do better in the second half of the season, tactically and we are going to do that."
Emery plays down fan boos
Emery played down a negative reaction from some Arsenal supporters after his decision to replace Lacazette for Ramsey was met with jeers.
The substitution was booed by a large number of fans, although Ramsey - who has been linked with the likes of Real Madrid, Juventus and Bayern Munich with his Arsenal career set to end when his contract expires - put the game beyond doubt with the third.
"I understand the supporters," he said. "I need to do my work. Tactically we thought in that moment we needed to change for more balance.
"Above all we know (Jean Michael) Seri is coming on and need a player close to him, not to let him play easily with the ball.
"Aaron Ramsey can do that and also help us in attack and scored. The reason is this. I need to do my work. And not maybe because every supporter can have a different opinion, tactically can do.
"But I need to do my work. Lacazette his work and he scored also. He helped us and it was a very positive reaction."
