I compared Morata and Lukaku for their minutes per goal or assist for the last three seasons in league and Europe. I count assists the same as goals because a goal created counts the same as a goal scored.
Lukaku: 14/15 Premier League 10G 5A 2876min 191.7min/GA; Euro League 8G 2A 632min 79min/GA
15/16 Premier League 18G 7A 3174min 127min/GA
16/17 Premier League 25G 6A 3267min 105.4min/GA
Morata: 14/15 Serie A 8G 5A 1321min 101.6min/GA; Champions League 5G 2A 744min 106.3min/GA
15/16 Serie A 7G 8A 1456min 97.1min/GA; Champions League 2G 2A 534min 133.5min/GA
16/17 La Liga 15G 5A 1331min 66.5min/GA; Champions League 3G 1A 165min 41.2min/GA
If Real were not fairly assured of getting Mbappe this year or next, and paying his transfer fee with the sale of Morata and James Rodriguez, they would be absolutely insane to sell Morata at any price. United getting him for 73m euros is good business IMO. If United had a proper player development program and player development strategy, like Real, it wouldn't be absolutely necessary to spend ridiculous transfer fees on quality players.