The 23-year-old striker had been criticised for starting the 2016/17 Premier League campaign slowly but kicked into gear with a fine hat-trick in Monday night's 3-0 win over Sunderlandat the Stadium of Light.
Of his return to form, the Belgian told Sky Sports that he was feeling so good out there that he could have scored a handful if luck was on his side.
"It was all about my fitness levels, they weren't as good as I wanted them to be," said Lukaku, who scored a double for Belgium in World Cup qualifying last week.
"But in the Premier League you have to be focused until the end and I got got good service from Yannick (Bolasie) and Kevin (Mirallas) at the end and managed to finish the chances.
"I think I could have had five really, I need to be more of a serial killer if I want to help the team achieve it's objective. We are in a good position in the league and we have to keep going."
Lukaku's three goals were scored in just over 11minutes 30seconds to become the 12th fastest Premier League hat-trick in history. It was also the first of the 2016/17 season.