Roberto Martinez, Belgium's manager, was Lukaku's manager at Everton for three seasons and is pleased with the progress the forward has shown.
"Everyone looks at Romelu Lukaku and forgets that he's 24," Martinez said on 'Match of the Day'. "He's already had two record transfers, to Everton (from Chelsea for £28m in 2014) and then to Manchester United.
"I've seen a big change in his maturity. He's got a little bit of everything. As a No 9 sometimes you play against them and they are a little bit predictable," he contined. "He's good with his back to goal, he's getting stronger. With the space he's unstoppable because he's got that strength and power but his outstanding quality is his finishing."
Martinez also said that Lukaku can still get better: "You get a good chance and he will score. He's still a young player and is someone that can develop."
Lukaku is likely to lead the line for Belgium at the World Cup this summer, and he already has 31 goals in 65 appearances for his country.