Romelu Lukaku faces a tougher challenge to reach the Manchester United records Wayne Rooney achieved, says manager Jose Mourinho.

United's all-time top goalscorer Rooney will make his first return to Old Trafford since rejoining boyhood club Everton in the summer on Sunday, live on Sky Sports Premier League.

Lukaku, 24, went the opposite direction, joining United for a fee which could rise to £90m, but despite scoring six goals in as many appearances, Mourinho explained why his new No 9 will find it hard to match Rooney's 253-goal haul.

I think Wayne spent probably 10 years of his career in a different Premier League than you have now," said Mourinho.

"A Premier League where it was easier to score goals not just because of the difference between the top teams and others but also because of the profile and the tactical interpretation of the competition.

League is much more defensive-minded, and much more difficult."

Mourinho believes his side are now demonstrating a killer instinct they lacked last season, having averaged three goals per game so far in the Premier League - an improvement on their 1.42 goal ratio in his first campaign in charge.

Zlatan Ibrahimovic was burdened with much of United's goalscoring responsibility in that time, but Mourinho wants other members of the team to continue to keep the pressure off Lukaku.