The Red Devils signed Lukaku in a £75m deal from Premier League rivals Everton in the 2017 summer transfer window.
The Belgium international scored 10 times in nine games in all competitions to make a brilliant start to his Old Tafford career.
Lukaku struggled to maintain his incredible form to raise questions about the Belgium international, particularly his record agains their top-six rivals.
The 24-year-old has rediscovered some decent form over the past two months or so, netting nine times and making two assists in his last 10 games in all competitions.
Firmino, meanwhile, has netted one goal less than Lukaku’s 15 in the Premier League, but the South American has netted eight times in the Champions League this season.
The Brazil international has played a key role in Liverpool FC’s run to the Champions League semi-finals ahead of their last-four clash against AS Roma.
Former Spurs star Waddle explained why he considers Firmino to be a better all-round forward than Lukaku.
“Lukaku deserves a mention because he has scored goals, which he was bought to do, but he does not make my six because he gives possession away to cheaply at times,” Waddle told BBC Sport.
“Roberto Firmino at Liverpool is a much more intelligent player.”
Manchester United are four points ahead of Liverpool FC in the Premier League table in the race to finish as runners-up to Manchester City,
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