Mourinho reckoned Lukaku deserved more support after an assist in each of his last two league games, the most recent of which led to the winning goal in last Saturday's 1-0 victory over third-placed Tottenham.
"I would like the supporters to explain to me why they don't support him so much because he gives everything and I think it is not fair when scoring the goal or not scoring the goal (he) makes the whole difference," Mourinho told the club's website. (www.manutd.com)
"I don't think it is fair at all. So, I'm a bit disappointed - but not with him. With him (I am) very pleased."
Mourinho's decision to withdraw Marcus Rashford, rather than Lukaku, for Anthony Martial in the 70th minute against Spurs attracted loud boos from the Old Trafford faithful.
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The Portuguese boss responded to critics by holding a finger to his lips after the final whistle but was puzzled by the fans' dissent.
"I really don't understand some reactions why," Mourinho added. "Are they Red Devils? Sometimes I don't know because they (Lukaku and Martial) work amazingly well.
"Sometimes he (Martial) starts the match and his contribution is good... the same as Rashford, the same as Lukaku."
Mourinho said defender Marcos Rojo was likely to return from his long-term knee injury after the November international break but set no return date for midfielder Paul Pogba, who injured his hamstring last month.
United trail local rivals Manchester City by five points at the top of the league table and host Benfica in the Champions League on Tuesday followed by a league trip to fourth-placed Chelsea on Sunday.
(Reporting by Aditi Prakash in Bengaluru; editing by Amlan Chakraborty)
