However, his 90 minutes against Chelsea marked the first time he has played a full game this month.
"He is a fantastic player, a player that is a modern midfield player, he can do everything", Mourinho said following the 2-1 win over Chelsea.
"That's the situation around me", Lukaku said, having scored his first league goal for United against a side now in the top eight.
Antonio Conte was interested in the striker before he chose United - and Lukaku insists he made the right decision.
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It lifted United back up to second place, above Liverpool, and opened up a six-point gap to fifth-place Chelsea with 10 games remaining. "We must be ready to fight, to be concentrated and to understand that this target is an important target for us but it won't be easy".
Both sides could also feel the after effects of their Champions League exertions in midweek.
The match came after weeks of rancor between Conte and Mourinho, who fired off increasingly edgy barbs at each other in news conferences.
"De Gea is a very hard goalkeeper to beat but we'll try to get past him in the return leg", said the defender.
The forward has taken a lot of flak this season, and did so again during this game, but the bare facts make him the standout player. Chelsea clearly benefited from the Brazilian being on the pitch alongside Eden Hazard and playing with a striker between them. Once they did, the equalizer arrived. From Lukaku scuffing a shot to Alexis Sanchez not hitting a brilliant pass from Anthony Martial first time, the miscues were galore.
Yet, the thing here is not about the style Manchester United play. The substitute gobbled up Lukaku's perfectly weighted ball, firing a header into the corner without any challenge from the Blues defence. The midfielder had only been on the field for 11 minutes. It's about how many games you play and how many goals you score.