The Belgian international striker was dropped to the substitutes’ bench for last weekend’s 2-1 win over former club Everton at Old Trafford.
But Lukaku is not even in United’s squad for the encounter at Dean Court.
United’s No.9 did not travel with the rest of the squad on Friday, when they flew south, after picking up an unspecified injury in training on Friday morning.
Lukaku picked up a muscle problem and it was decided not to risk him, with Juventus on the horizon.
“In training yesterday, after the press conference, he felt something that needs to be studied in the next couple of days,” said Jose Mourinho.
“So Lukaku has not come and we decided to start with Alexis.”
Lukaku has been suffering a goal drought since opening the season with four goals in his first five games.
Despite the 25-year-old netting regularly for his country, he is nine games without a goal for Mourinho’s side – even though he has played the full 90 minutes in all but one of those games.
With confidence low, he was pulled from the lineup against the Toffees, but had been in contention to return against the Cherries.
However, after picking up a problem on Friday, it was agreed with medical staff that he would not travel and miss today’s game.
Anthony Martial and Alexis Sanchez lead the United attack, with Marcus Rashford dropped to the bench after starting United’s last five games but failing to score.