The left-back was injured against his former club Southampton on Monday, and both he and his replacement at Old Trafford that night, Brandon Williams, were subsequently ruled out of the win at Crystal Palace on Thursday.
However, the boss hopes homegrown product Williams will have recovered from the facial injury he suffered against the Saints in time to be in consideration this weekend. Tim Fosu-Mensah filled in capably at Selhurst Park, and already has experience of playing in a last-four tie in the competition, against Everton in 2016.
“Luke will be out,” the boss told us. “I can't see him recovering.