The Belgium international has scored nine goals in the Premier League this season following his big-money switch to Manchester United from Everton in the summer transfer window.
The Red Devils coughed up a £75m transfer fee to sign Lukaku ahead of defending Premier League champions Chelsea back in July.
Lukaku scored 10 times in his opening nine games before he managed four goals in his next 15 appearances to raise questions about the 24-year-old centre-half.
The summer signing’s big-game mentality has to be questioned following his failure to find the net against Arsenal, Chelsea, Liverpool, Manchester City and Tottenham Hotspur.
Former Republic of Ireland defender Cunningham believes Lukaku has plenty of room for improvement at Manchester United.
“I think he’s still developing. He needs to mature. He isn’t the finished product,” Cunningham told Irish radio station Newstalk.
“He was isolated but he still has to make more of a nuisance of himself. He has to be better in possession of the football. His game has to be better.”
Lukaku scored 25 goals in 36 games in all competitions during his final season at Everton before his big money move to Manchester United.
The Red Devils haven’t won the Premier League title since Robin van Persie scored 24 times in 38 games in their successful 2012-13 campaign under Sir Alex Ferguson.
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