Romelu Lukaku has said his "painful" first stint at Chelsea turned him into "the complete package".
The 28-year-old Belgian returned to Stamford Bridge this summer, 10 years after he was first snapped up by the Premier League side as a highly-rated teenager.
Lukaku failed to make an impact and was loaned to West Brom and Everon.
But the Belgium striker is back at the club as one of the most fearsome goalscorers in European football.
"It was painful and helpful. But it gave me the mentality and the mindset that I needed to become the player I am today.
"The team was very good but nobody saw the extra work they did after training sessions. As an 18-year-old, I saw every day how much the players worked on their craft.
"I told myself 'when I'm not playing, this is what I'm going to do', and it basically just became a lifestyle," Lukaku told Chelsea's official website.