"The offensive side of the team on Monday was great, much better in the second half," Koeman said at Thursday's pre-match press conference.
"For the team performance, don't make an eight and next week a four. But of course it's football.
"One day, you play better than the other. But if we talk about Lukaku, he is 23 years old. He did not have the pre-season he needed to be 100 percent. Now he is 100 percent.
"It's always a boost. Strikers live for scoring. Lukaku is always a scoring player. But we need to support the boy to give him the chances to score."