Lukaku made a blistering start to his United career after joining from Everton, scoring 11 goals in his first 10 games, but has now gone six matches without finding the net.
Mourinho made a point of defending Lukaku after Saturday’s 1-0 win over Tottenham Hotspur, but when United won a second-half penalty against Benfica on Tuesday, he instructed Daley Blind to take it.
“I don’t understand why everyone asks me the same (question),” Mourinho said when asked about his decision in his post-match press conference at Old Trafford.
“I’m paid to take decisions, good or bad. In the first half, the decision was bad because the player I chose to take the penalty (Anthony Martial) missed.
“In the second half I made another decision and I don’t see the drama. The penalty taker for this match was Martial. He took in the first half and missed.
“He was not even on the pitch when the second one was (awarded). And then I have to make a decision. Romelu wants to take and had the personality to take the responsibility. (Ander) Herrera the same.
“But my decision, based on what we did in training in the past couple of days, was Blind. The players respected my decision. That’s it, end of story.”
Martial’s 15th-minute penalty was turned behind by Benfica goalkeeper Mile Svilar after Douglas had been penalised for handball.
But Svilar, who had carried Marcus Rashford’s free-kick into his own net to gift United victory in the reverse fixture, proved Benfica’s fall guy again when Nemanja Matic’s strike hit the post and bounced into the net off his arm.
Blind settled matters in the 78th minute by drilling a penalty down the middle of the goal following a foul on Rashford by Andreas Samaris.
Mourinho had reacted to Saturday’s win over Tottenham by performing a shushing gesture and after Blind scored he clacked his fingers together to apparently signify chatter.
Asked to explain the gesture, he replied: “I don’t remember.”
– Hapless Svilar –
United’s fourth successive Group A victory left them six points clear, but CSKA Moscow’s 2-1 win at Basel means Mourinho’s men require one point from their last two fixtures to rubberstamp a last 16 berth.
Mourinho played down the importance of finishing top of the group, which theoretically guarantees a kinder draw in the next round.
Yet with United having graced the tournament only once in the previous three seasons, he would like to see his team win all six of their group matches.
“Sometimes a team finishes first and gets an extremely difficult draw,” he said.
“Other times you finish second and you are so lucky to get some team that nobody expected to finish first.
“We want to win the maximum possible points. I also know we haven’t qualified yet. We are going to try to win the next two matches and finish with 18 points.
“The last time Manchester United was in the Champions League (in 2015-16), they finished third and went out. So the year we return, it would be nice to do 18 points.”
Benfica head coach Rui Vitoria defended 18-year-old Svilar after his second hapless moment against United in as many weeks.
“Svilar is still a junior. He’s still 18 years old,” said Vitoria.
“He did a lot. In today’s match he showed what he’s capable of. We know what Svilar is capable of and he played well.”
Vitoria was phlegmatic about his team’s chances of keeping their European season alive after a result that left the Portuguese champions six points adrift at the foot of the group.
“There are still two games to play,” he said. “With six points, you can go to the Europa League, stay in the Champions League or get nothing.”
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